What's not solid about it?
Blink Blink
The fact that you are saying nothing? Look, I said we can drop it, and we can, but in response to "Does Francis exist even if he isn't that specific guard" you have said "At some tables he might, at others he wouldn't" All while spending an awful lot of words telling me the rules say nothing about it.
That's a non-answer, there is nothing there to discuss. Some tables do, some tables don't. It is true, but it doesn't give us anything to talk about, it is a deflection.
Always right in that this is what the character thinks, anyway, since the rules say the player determines what the character thinks. Those thoughts themselves might be wrong.
And if you decide they are wrong about the existence of an entire person, what does that say about the Character's mind? In fact, since the player cannot choose for the NPC to be real, if they DM chooses that they are not, then the Character has an entire made up person in their head they believed to be real. Why?
You can say "Well, that answer doesn't matter to me" but as the DM it does, because you are the player's window into this world. If a player doesn't know where these lines are, because they have absolute authority over their character, they can end up with a character who is completely delusional, constantly wrong about facts of their own lives. And if the player didn't come forward with that as a concept, but is instead dealing with it because of the DMs rulings, that can become an issue at the table.
"Buying scrolls of Thunder damage spells" does not necessarily require knowledge on the part of the character of the weaknesses of earth elementals either, even if the player knows a battle with such creatures looms.
... So, to be clear. A player stating "I am going to buy scrolls with spell that deal thunder damage because I know we are fighting earth elementals and they are vulnerable to thunder damage" does not require knowledge of earth elementals being weak to thunder damage...
Because, I did state they were buying them under that assumption, therefore it was the driving motivator behind their decision. I didn't say they bought them because they were the cheapest spells in the store, or because they liked loud booms, I said it was because it was utilizing knowledge of a specific weakness. And your counter to that is that they don't neccessarily have to be buying them to utilize that specific weakness.
As far as inconsistencies in stories go, there have been at least 3 origin stories for Mind Flayers that I know of (and that's before we even finished 2e) and I wouldn't be surprised to find recent editions have introduced more of them, or that 2e settings I'm not that familiar with (Planescape, Spelljammer) had their own backstories that weren't completely congruent. Beyond that, I'd never assume that a particular DM was using one canon or another. Running a game in say a Marvel or DC universe would be equally ambiguous. The comics are full of retcons.
Completely beside the point. First of all, I said "Are created" not "Were created". Now, I will grant, easy distinction to miss, but I was talking their breeding habits, not their origin story. I can claim that, but either way the retcons of editions have nothing to do with the point we are discussing.
If a poor street rat knows a bunch about Mind Flayers because his player knows a bunch about Mind Flayers, and he chooses to act on that knowledge in character by relating all the stuff he knows, that's nothing I can do anything about. I can't tell a player how to play their character, and I can't make players forget what they know. They'll have to make choices about what they are comfortable doing. If we need to establish how he knows it, well, that's never that hard to do and in my experience often makes for fun story hooks especially if the player is willing to let me run with that.
First off, the bolded part is false. There are things you can do. Maybe not a lot of productive things, but things nonetheless.
For example, I encourage my players to ask me, just like I ask my DMs. I don't find that shameful or DM powertripping or anything, it just is useful. That way if they are going off of info in the MM that I changed, I can let them know that isn't what I'm using. Sometimes things make perfect sense, sometimes I need a second to think through how they could know something. And, I never try and have people hide mechanics, like resistances and vulnerabilities, those aren't the things I'm concerned with.
And frankly, it rarely comes up at all. But, as the DM, I am the curator of the story, I mix the player's various threads and make a whole, and that might mean setting limits on player knowledge, especially when the lore is meant to be revealed as part of a big plot. Sure, I can't wow the veteran player who knows the secret, but that doesn't mean they should ruin the fun for everyone by blurting it out when their character has no reason to know.
Maybe so, but that's the players choice how to play it as far as I'm concerned. If the player wants to play this as, "Don't worry Johnny, this will only hurt for a little while...", that's the player's decision, and the fact that everyone is taken aback by this reaction might well be interesting. I prefer not to tell players how their character acts. The player has little enough control over the game as it is with me stepping on the one prerogative that they unambiguously have.
But am I overstepping by saying they feel a "dawning horror" over the reveal? That's the only thing I'm saying that you aren't. I'm not going to narrate how they act, but, is it too much to give a nudge in the logical emotional direction?
Some people say yes, but I don't think so. I don't think I'm overstepping.