1. Ability scores generation method?
In AD&D, usually 4d6, drop the lowest, arrange to taste
2. How are death and dying handled?
It depends on the game. No one dies at zero, characters will die according to their constitution, stamina, and other factors. Conan the barbarian,as an example, is not going to die at -1 or -2, though he will be knocked unconscious at 0, as everyone else.
3. What about raising the dead?
You must be joking. Only Jesus Christ was able to raise the dead.
And he could perform such a MIRACLE because he was the Son of God....
4. How are replacement PCs handled?
Same level as party average, new character fortuitously met at a convenient point in the adventure.
5. Initiative: individual, group, or something else?
It is obvious you are talking about AD&D here. Well, i love the detailed initiative in ad&d, individual initiative modified by weapon speeds, dexterity, etc and anyone rolls his own.
But this process takes too long, so i opt for group initiative (a single roll for the entire party) modified by each character's abilities and weapons and casting time.
6. Are there critical hits and fumbles? How do they work?
Sure, not only are there critical hits. If you can hit an opponent with a 16, rolling a 18 is not the same thing. Rolling a 20 can be devastating.
7. Do I get any benefits for wearing a helmet?
Yes, only if someone scores a critical on you (18, 19, 20..)
8. Can I hurt my friends if I fire into melee or do something similarly silly?
9. Will we need to run from some encounters, or will we be able to kill everything?
Running, or using strategy is as important as killing.
10. Level-draining monsters: yes or no?
Yes, that kind of denizens exist. Firstly, vampires. it is your fault if you decide to go an adventuring.
11. Are there going to be cases where a failed save results in PC death?
Yes and no, but here it depends a lot on which game i am playing, especially how many kind of saving throws exist in that game and against what they are intended to be.
12. How strictly are encumbrance & resources tracked?
I used to hate this aspect, i don't abide strictly by the rule, but i use it of course, no one can ignore that.
13. What's required when my PC gains a level? Training? Do I get new spells automatically? Can it happen in the middle of an adventure, or do I have to wait for down time?
You advance at the end of the adventure, otherwise there'd be too much to keep track of.
14. What do I get experience for?
Treasure, monsters, roleplaying, end of goals, successful cast of magic, travelling, etc..
15. How are traps located? Description, dice rolling, or some combination?
Sorry, don't remember now how i handled this in AD&D.
16. Are retainers encouraged and how does morale work?
Rarely used by my players. Morale is almost never a random mechanic, as isn't influence chance exerted by PC's on others.
17. How do I identify magic items?
A spellcaster might determine it, though it's not automatic.
18. Can I buy magic items? Oh, come on: how about just potions?
No, see question 3. You can stumble upon that. How is ridiculous that there may be a magical shop? Magic would cease to be "magic".
19. Can I create magic items? When and how?
Yes, but not with just time and money. Trying to accomplish such an endeavour entails MUCH, much more than that.
20. What about splitting the party?
If the party splits, it splits. The Game master just can't forbid such an happening to take place.