OK, I have some more time now, but not much! I will start by saying the monster building rules are not, IMO, for new DMs. They are for experienced DMs. In general, I think any of the rules for creating new things and altering the rules of the game in the DMG are for experienced DMs. New DMs should, IMO, stick to the MM for monsters, the rules in the PHB, and how to play guidance in the DMG. Once you start homebrewing that is, IMO, for experienced DMs.
So I reject the notion that the monster building guidelines are for new DMs. Additionally, despite the title in the DMG, the are not quick. There are 20 steps listed in the process with many sub-steps within those steps. These are not guidelines for the new / inexperienced DM.
If that is your criteria (that they be easy for new DMs) then I agree they are a failure. That is not my criteria. So I generally disagree with your premise and that is probably as far as we need to go. However, I do have a few more comments.
The Boglins are low level. They're fey but weak ones. So you give them a CR of 1/8, which is pretty much the lowest possible. You're not sure about the math. And the Driderlings are meant to be much stronger at CR 1/2
- CR 1/8 Boglin: Proficiency +2, AC 13, 21 hp (avg), +3 to hit, 2-3 damage
- CR 1/2 Driderling: Proficiency +2, AC 13, 60hp (average), +3 to hit, 6-8 damage.
Holy pinatas Batman! You are looking at 60hp for a CR 1/2 monster.
Those are boring monsters, but technically functional. I will say this is not how you're supposed to make a monster (simply picking numbers of the table). But they technically function properly.
I am by trade a designer (architect) and I have trained to design in art and building. Design is not easy and takes a lot of work. That applies to architecture, art, and monsters. I don't think simple picking some numbers off of a table will every make a good monster. That is why it is not, IMO, an task for a new DM. It takes experience / training to be good at monster design.
But! It gets worse! You want to spice them up a little so give them powers from p280. The boglins are creepy fey tricksters. So let's make them good at hiding by giving them Invisibility, and good at trickstering by giving them Charm, Read Thoughts, Mimicry, and both with Shapechanger. Oh, and because we're feeling generous let's let them teleport to help with both. You know how much this changes their CR by? Literally nothing.
A few things here. Some of my appreciation for the 5e monster design guide is based on context. I didn't play 3e, but 1e, 2e, and 4e gave no guidance on the items in bold either. 5e does in fact give guidance on a lot of monster features and traits, more than any previous addition. So some of my praise for the 5e builder is in relationship to previous ones.
Now, I also disagree that these things don't affect CR and I agree the guidelines could be better here (and I hope they are in 2024). What you listed are all spells or spell like. Step 13 has a section on spellcasting, but doesn't go quite far enough in explaining that you can convert those spells to damage (the WotC designers have explained this in interviews - but it is not in the DMG). Basically you take the spell that inflicts just that effect or condition and use the damage by spell level for the "effective" damage. If that is higher than the other attacks, it affects the CR.
And Web tells you "Increase the monster's effective AC by 1" - so how much does that change the CR by? We go back to Step 4: Final Challenge Rating and look under "Defensive challlenge rating". It states "If your monster's AC is at least two points higher than that number, adjust..." It isn't. Web only adjusts it by one so it too is free.
No that is incorrect. It could be free or it could cause in increase in CR. It depends on where everything else is. So if you are CR 1, but the calculation was really a 1.25, the +1 to effective AC could push the total CR up to 1.5 which is typically round to 2.
However, again there is an art to this. It is unlikely any guide can account for the breadth and depth of human imagination. There will need to be judgement calls made and that is, again, where experience comes in handy. Not for new DMs!
It's entirely possible that the table works (a) when you start above level 2, (b) when you ignore the Quick monster creation rules, and (c) when you have a good feel for what you want to create. But this needs a significant overhaul for the most important group - new DMs trying to create their first monsters.
No the table only works when you carefully follow the creation rules they explicitly do not work separately. I hope for only slight tweaks to improve guidance in a few areas for the 2024 DMG.
However, I would be fine with another truly: Quick Monster set of rules / tables for new DMs. It would; however, not be able to handle the nuance you seem to want. That is not something new DMs can do IMO.