You are correct that it doesn't say anything explicitly about corners. Neither did I.
Part you didn't quote above from pg 191 says "A creature's space also reflects the area it needs to fight effectively." When combined with the rules for squeezing into a smaller space (pg 192) where you can not fight effectively, pretty much shows that you are, for game mechanics sense, in your entire space unless you are squeezing into a smaller space.
I can Squeeze (taking the penalties) to have a smaller space, and if that space is now outside the area of effect - then I'm outside the area of effect.
If I'm not Squeezing, then by the rules I don't have a smaller space.
There would not need to be rules for reducing to a smaller space if it was something that you could do on-the-fly and for-free to avoid area of effects.
Is a creature's space a gamist simplification that doesn't match well with how much space people take up in reality? Sure. That's why realism-based arguments aren't particularly relevant - the game has already put them aside to make something easily playable. I'm not saying this to support my argument (so you don't try to refute it and think it affects the stance above), I'm saying it because I understand where you are coming from. It doesn't make perfect real-world sense. Neither do all doors being 5' wide, but we still see that everywhere from the same design decision.
So in other words
- A creature doesn't occupy the whole space, but he does.
- A lightning bolt is instantaneous so it doesn't matter that a person was in it's path a moment ago but then moved to the side because they were doing a feint.
- The lightning bolt just hangs out for the entire round while the creature is forced to leap into it because they must occupy that 2.5 feet of space at some point.
- Assume that in the middle of intense combat a wizard has the time to draw an imaginary line exactly down the middle of the hallway, not off by even an inch or two while an ogre is trying to smash in their face.
- The five foot wide lightning bolt must be imagined as being 100% exactly the same width for the whole length, making it a lightning "tube". No imagining a jagged line where the 5 ft wide is an average approximation. All the drawings we've ever seen of chaotic lightning bolts are lies.
- All rules must be approached exactly as if D&D were a board game that must be literally interpreted or you're doing it wrong. I'm not allowed to look at how big a 5 ft square is in real life because "game"
- The designers really meant to make the lightning bolt 10 ft wide because there's no reason to not thread the needle every time. It's just an intelligence check to see if you can figure out the loophole.
By this logic, all missiles fired into a 5 ft square space should always hit. They may ineffectively bounce off armor, but they will always connect with something.
Anyway, have a good one.