2. why do we get stats for the underdark elf (drow) but not the underdark dwarf (duergar) or the underdark gnome (svirfneblin)?
They’re in a different book. I think the real question is why Drow are in the PHB instead of the EEPG, and I think the answer is Drizzt.
3. why isn't there a skill for constitution? it could be called stamina.
What would such a skill be used for?
4. i see there's different damage rolled when using a versatile weapon two-handed. does this mean the 1.5 strength bonus is no longer added to damage? as i recall, in 4e the damage bonus was a flat +1.
Correct.
5. what happened to the paladin's divine grace?
I’m not sure what you mean.
6. why do characters get an ability score improvement at level 19 rather than 20? is this an attempt to avoid dead levels?
I think it’s so that the classes can all have unique and interesting capstone abilities at 20.
7. why do fighters get 2 extra ability score improvements (at levels 6 and 14) and rogues get 1 extra (at level 10)?
...because those are features of those classes? I feel like I’m missing something that’s going into this question.
8. why are the sections on preparing and casting spells so complex? suddenly, i'm longing for the good old days of simple, vancian casting.
I’m guessing you played 3.5? Everyone is a sorcerer who can change their list of known spells every day.
9. why are there so many different ability-use systems (at will, regain after short rest, regain after short rest or long rest, point pool use, superiority dice, etc.)?
5th edition went through a pretty rigorous public play testing and survey process, and surveys revealed high demand for a variety of different resource systems.
10. can finesse weapons be used two-handed? would there be any benefit?
There’s nothing stopping you from wielding any weapon in two hands, but there is no special benefit for doing so unless it has the Versatile property. There are no weapons with both Finesse and Versatile in the PHB, nor are there any with both Finesse and Two-handed. Technically there’s no reason a DM couldn’t make up a custom weapon with either of those combinations of properties.
11. i'm a little confused about short rest healing. let's say bob the 8th level fighter takes a short rest. he can roll 8d10 healing during this rest, and these dice are replenished after a long rest. is that correct?
More or less. Bob has 8 of a resource called Hit Dice. On a short rest, Bob can spend any number of his Hit Dice. For each Hit Die Bob spends, he rolls a d8, adds his Constitution modifier to the result, and regains that many hit points. On a long rest, Bob regains all of his Hit points and 4 Hit Dice.
12. what's the math behind advantage/disadvantage? does it correspond to a simple +2/-2? or is it way more complicated than that?
Way more complicated. It works on a bell curve based on the number needed on the die to succeed. If you need a natural 10 or 11, Dis/Advantage is roughly equivalent to -/+5 If you need a natural 20 or 1, Dis/Advantage is worth a little less than -/+2 (I think).
13. why don't fear effects target charisma saves? one aspect of charisma is confidence, after all.
Because they target Wisdom saves. Why WotC decided to have them target Wisdom
Saves instead of Charisma is anyone’s guess. I think they wanted Dex, Con, and Wis saves to be more common than Str, Int, and Cha saves.
14. do critical hits need to be confirmed? i see no mention of this, so i really hope it was dropped...
Nope, it was dropped.
15. what's with all the omitted spells? no prismatic sphere?
They’re only “omitted” if you assume that Because a spell existed in a previous edition, it must exist in all subsequent editions.
16. i've heard there is a pdf of revised classes available for download somewhere. where can this be found?
I’ve never heard of that.