>what damage should the weapons do?
Per BX/LotFP went fine in my group.
>should skills like ranger tracking or rogue lockpicking be % based, d20 based? or d6-roll-under based? (like LotFP)?
Again, LotFP went fine. Roll under attribute checks for non-specific situations also went fine. Failing on 20 breaks off the routine, plus with 3d6 and BX modifier scale it makes more sense than figuring out DCs.
>what should happen at 0 hp?
Death. Alternatively, LotFP rules or various dismemberment/wounds options. I opt out for the latter with death if you're hit and damage goes further than your first hit die value.
>what hit die should fighters have?
LotFP/BX hit dice are fine, although feel free to AD&D stuff for them.
>how should I do spell levels?
As default, or Wonder & Wickedness (worked really well in my group)
>how do I allow for cool spellcasters without them being overpowered?
Wonder & Wickedness is cool, but you'll have to work with the spells quite a lot. I did and I don't regret it. Also look up The Nightmares Underneath.
>how to make smite evil work?
Smite Chaos, use one axis alignment (L/C/N)
>how to make XP work in a way that makes leveling up gradually grow slower?
On your own here. Too long for me to explain from phone plus all of my stuff is in Russian. Hope somebody helps.
>how to make attack progression work?
As per BX.
>HOW DO I DO SAVES FOR CHRISTS SAKE
Personally, single save is cool for those. One of my group was fine with LotFP saves, the other autistically screeched even after I explained the concept. Single save is fine, class bonuses at your leisure. Fighter gets bonus against Death, Magic-User against Magic (wewlad) and so on.
Don't have time to review your stuff, sorry about that.