* scrap 1E weapon speed and reach - this is not "fun", strictly speaking
I actually liked these. Its simply a initiative modifier for how big, or small, your weapon is, and further differentiates how "Good" the weapon is. As for reach, I only worried about that for pole arms. IE give first attack upon closing to those with pole arms readied. Or those with bows/crossbows readied.
* scrap race/class stat requirements - life is short, let them play what they want
Yep, did this. Guess what? I was still playing 1E! For over 5 years!
* scrap race restrictions on class - see above
I did this too. Even allowed Dwarven Paladins. GASP!!! I was still playing 1E.
* scrap bonus XP based on high stats - the less bookkeeping exceptions the better
I never did this, but don't see it as destroying anything, except extra book keeping.
* roll initiative once at the start of combat - ties go to the player
Sounds good to me.
* combat spell durations last the whole encounter - no more duration bookkeeping
Another good one! I didn't start doing this until 2E, but it works well!!
* 4E alignment system (I understand this will nerf some 1E spells - meh)
Just turn all reference to "alignment" to "enemy" and it works wonderfully.
* 4E minions (this is a cinematic foil - obviously would have to fudge XP to compensate)
I have always had, effectively, minions, so this is no big change for me.
* triple starting HP
I don't think I would do this, but your group will find out how much you like it soon enough.
* scrap wizard spell pre-selection, let spell allotment be selected and fired ad hoc
I played in games like this, and it worked without destroying game balance until about 10th level, which was the highest that DM wanted to run his games anyways. So it worked.
* let non-class NPCs cast "rituals" - this is a good plot device but the mechanics justifying it would have to be handwaved.
Sounds OK to me.
Maybe give them an at-will, DEX-based Magic Missile attack a la 4E?
Sure, I did this in my C&C house rules, so it will work in 1E too.
1E fighters seem pretty lame - any suggestions on making them appealing? IIRC, weapon specialization and proficiencies were introduced in Unearthed Arcana but I prefer the simplicity of the original "permitted weapons" for each class. Still, that leaves the fighter kinda dull. Would giving them more combat abilities result in gross imbalance?
Well, don't be afraid to give them cool magic weapons and armor. That helps, a lot. They can already grapple, charge, etc...
I've never met a player who enjoyed playing a cleric and will not force a player to suffer this. But I'm not sure how to make 1E combats feasible without one (other than to inflate starting HPs); any suggestions would be gratefully received.
I allowed two things in 1E. Spontaneous casting like your allowing mages, which in turn allows them to cast healing spells when needed. Like they do in 3E. So I was waaaayy ahead of my time in regards to much rule design for D&D.
Also, can anyone recommend how to make combats easy to calculate? I seem to recall something about strangely complex "to hit" tables for weapons. If possible, I'd like to somehow incorporate an easy calculation of hitting-vs-AC.
Yeah, use the formula given for 2E or C&C.
Power inflation will happen, but as long as the monsters/NPC's get it too it stays largely balanced. As balanced as 1E can be.
Also realize the magic item treasure charts are intentionally designed to balance the game too. So if this ends up being long term keep that in mind.
If it is only for a few sessions, no need to worry about it.