That's not in opposition to anything which I have said.
I also don't disagree with a lot of what you said. But I do want to say that while I respect your position, I believe that my opinion on the meaning of the text is equally viable. I want to make it clear what these varying interpretations are so a DM can confidently make a decision as to how they want to run it.
I also have a preference, and were I a player rather than a DM I would argue the heck out of it with my DM, because I believe it is a significant nerf to both pact of the chain and an interesting feature (including for non-warlocks) in the MM to not interpret it as I do.
That side-bar has zero interaction with the other side-bars, and demonstrates nothing except that NPCs are not held to the same rules that PCs are - most strongly evident in mentioning "...or some other Tiny monster, such as a crawling claw..." which is not available as a familiar for PCs.
To get nit-picky (and I believe reasonably so in context) the sidebar in question doesn't mention NPCs or PCs at all, either in the text or title. It is on the same page as the NPC Mage which might imply that it is intended only to apply to NPCs, but that is pure interpretation by assumption, the same as my interpretation by assumption that two sidebars which discuss similar issues are compatible with each other.
My interpretation is entirely the opposite. For instance, I would say that the mention of other Tiny creatures is a suggestion for the DM to open up variant familiars even further (though there is no indication that such familiars would include the special magic resistance transference trait, because it isn't mentioned in those creature's entries). Rather than the 3e version where you needed a feat to get a special familiar, 5e allows special familiars via negotiation with a creature at the DM's option (which is conceptually in the same ballpark as to how 5e doesn't have wealth by level or magic item by level assumptions, so the DM's can hand out powerful items at low levels if they so choose, and no character resource needs to be spent for that to be okay).
That side-bar also doesn't actually state a requirement for the NPC to be able to cast find familiar in order to have a familiar - it's mention of find familiar is that any spellcaster that knows it is likely to have a familiar. The two are significantly different.
Nothing says that an NPC can't have met a pseudodragon, imp, or quasit, and had that creature offer up the service as familiar mentioned in those creatures' respective variant side-bars.
It doesn't make a lot of sense to me that the character (which as mentioned has not clearly stated it is only discussing NPCs) would have to know the spell, but not use it, in order to get those familiars.
I ignore the sources of information circumstantially, such as when one source is written without requiring any interaction with another source.
So I don't ignore the find familiar spell when working out how the Pact of the Chain feature works because it specifically calls out interaction with the spell, but I do ignore a side-bar about NPCs having familiars when not dealing with an NPC, or the variant familiar side-bar when it is the unrelated find familiar spell, rather than the creature's choice, resulting in the creature becoming a familiar.
That is a reasonable justification for assuming that the familiar from Pact of the Chain doesn't get magic resistance. I do believe the text best supports that particular assumption. However, I also believe it creates an annoying forced-choice scenario for the Pact of the Chain, if you interpret the variant sidebars as I do.
Focusing away from Pact of the Chain and on to the other stuff, here is what we have
without interpretation:
1) Certain PCs and NPCs (such as the archmage and mage) can cast the
find familiar spell.
2) The PHB tells us how that spell functions.
3) A sidebar in the MM says that characters (without specifying PC or NPC) can have variant familiars, which includes a list of non-exclusive examples beyond what the spell itself lists.
4) In order for those characters to have such familiars, they must be able to cast
find familiar
5) Three specific sidebars in the MM say that spellcasters (without specifying the
find familiar spell, nor PC or NPC) can have them as a familiars that grant special abilities.
6) In order for those characters to have such familiars, both parties must willingly enter a contract and form a magical bond.
7) Some of said variant familiars overlap with the familiars mentioned in #3.
8) The warlock of the Pact of the
Blade allows a character to either magically create a special item, or created a bond with one that already exists. In the latter case, that bonded object gains some of the benefits of the created item, including the ability to be dismissed and summoned/re-created, not normally possessed by such an object, removes none of its existing capabilities, but does not allow it to be changed into a different form.
Everything else is interpretation. So the question then comes down to what basis for interpretation we are using. My basis for interpretation is my well-educated guess on basic design philosophy for the edition, as well as precedent set by other similar rules concepts and interactions.
Here's how I see it and why:
A) Because design philosophy intentionally walks a middle ground between 3e's "PCs and NPCs follow identical rules" and 4e's "PCs and NPCs follow completely different rules", plus the complete lack of any disambiguation of the words "character" and "spellcaster" (words which the 5e rules use for both PCs and NPCs) in the text of the material in discussion, I assume that the rules are best interpreted as applying to both equally. (A hard line position of non-parity in rules could assume otherwise, but that would be pure assumption or intentional DMing interpretation choice.)
B) Either there are two ways described that a character can gain a variant familiar, one being by being able to cast the
find familiar spell and the other being by making a contract and magical bond with that creature, or these are referring to the same method.
b1) If they are referring to two different methods, we have very little information on how the first method works. In either case, we have no information within the sidebars on what it means for the creature to be a familiar.
b2) If they are referring to the same method, they can be seen as entirely compatible by assuming they are each only addressing a facet of the special situation. To gain a special familiar, you enter into an agreement with such a creature, and you must also be able to cast
find familiar. In this case, a reasonable assumption is that the casting of the spell is the method used to form the magical bond.
C) There are no instructions as to how the non-overlapping creatures from #3 and #4 are actually made familiars. If B1 is accurate, it doesn't provide us with any answer. If B2 is accurate, it makes perfect sense to say they follow the same procedure. In any case, since there is no mention of them gaining/granting the special features in the variant sidebars for specific creatures, it seems most plausible to assume that no such features are granted.
D) There is no indication in the MM as to what it means for the variant creatures in #3 or #5 to be familiars. It simply says they are familiars.
d1) This could be a meaningless term implying simply that they hang out with the spellcaster. Ie, they become a companion.
d2) But I find it more reasonable to assume it should mean that they become actual familiars. Familiars are described in the PHB under the spell
find familiar and in no other place in the rules.
E) Since the
find familiar spell specifies summoning a spirit (celestial, fiend, or fey) that takes certain forms, while the rules for variant familiars refer to actual already present creatures of their own types (which can include dragon and undead), interpretation is required to determine how
find familiar might interact with such beings. My interpretation includes the following: Since it is an existing creature it retains its own type. It can attack if its statblock specifies that it can. If reduced to 0 hp it disappears, but can be re-summoned (restored) by casting this spell again. You can dismiss it and recall it from the pocket dimension. Since it isn't a spirit, you cannot cause it to assume a new form by casting this spell again. Otherwise, it uses the rules for a familiar. The reason I assume that it can be restored to life by casting the spell again, and that it can be placed into a pocket dimension are:
F) At least one of the special familiars (pseudodragon) is described as being a "superior familiar". However, the durability of such a creature in the MM is highly limited. It seems extremely unlikely that anyone would consider a familiar that dies so easily to be "superior" to the standard familiars in the spell description that can be put into a safe pocket dimension, and easily restored if slain. By interpreting the variant familiar as receiving such features, the familiar is actually superior to the standard familiars, while retaining the interesting limitation that it can leave you if it wishes (for imps, you must violate its contract for it to leave).
G) The warlock Pact of the Blade provides conceptual design precedent (see #8) for exactly the features that I am giving to the special variant familiars to make them superior familiars.
Let's see how these interpretations interact with Pact of the Chain. Without interpretation.
9) The familiar is gained by casting the
find familiar spell. You have four specific alternate options for its form.
Pact of the Chain interpretations:
H) The familiar is still a spirit and functions identically to normal familiars gained solely through the spell, except for receiving the stat block elements of those particular creatures. Because of that, as described, it cannot attack unless you use your class feature to give it an attack.
I) It does not receive the special benefits from the MM sidebars for some of those creatures listed, as they rely on a contract with an actually present being, rather than a summoned spirit.
J) Despite G and H, I choose to override those and grant it both the ability to attack normally and the special sidebar features of the actual creatures in the MM.
K) I do this because I feel that design intent of the PHB was to make the familiar gained through Pact of the Chain superior to any other familiars. Interpreting the MM (that was completed months after the PHB) as I have would make the Pact of the Chain familiar inferior to one gained by contract. Therefore I give it all of the abilities of any form it takes, including the special magical features in the sidebars.
And overall:
L) None of these interpretations (variant familiar general rules or Pact of the Chain) make the familiar or the class feature overpowered.
M) I believe a rank of familiar power and utility is implied in the game, such that: Pact of the Chain familiar > non-Pact of the Chain spellcaster's special variant familiar > non-Pact of the Chain spellcaster's standard familiar. This interpretation supports that.
N) The uninterpreted information given on familiars as described in #1-9 is insufficient to provide solid rules related to variant familiars--therefore
some interpretation is unavoidable.
O) This overall interpretation of familiars in the game is:
o1) As reasonable as any others I have been exposed to, and more consistent and comprehensive than some
o2) Has no negative impact on the game that I can think of, and
o3) Makes the game more fun and interesting.
So, basically, a DM has to interpret the familiar rules whenever you throw the MM variants in the mix, and out of the most reasonable interpretations, you might as well follow whichever one makes your game more fun. Since I believe that my interpretation is more fun than any of the others, I share it with other DMs as a sort of "public service interpretation" on behalf of players everywhere.
