Consulting only the AD&D core books and Sage Advice from Dragon or similar sources, does anyone have a definite answer on these?*
Dexterity adjustments for Saving Throws
I don't recall ever using this, but a player pointed it out:
Defensive Adjustment refers to the penalty or bonus applicable to a character’s saving throws against certain forms of attack (such as fire ball, lightning bolts, etc.) due to dodging ability. It also applies to the character‘s parrying and/or dodging ability in missile or melee combat; in this case the penalty subtracts from the armor class (q.v.) of the character, making him or her easier to hit, while the bonus adds to the defensive value of the character’s armor class, making him or her harder to hit.- PLAYERS HANDBOOK, p. 11
Like I said, I never recalled that coming up in play back in the day. I guess we never noticed it.
I can pretty easily make some judgment calls about when to apply it.
But does the bonus stack?
Halflings get a bonus to saves vs. Rods, Staves, and Wands, and Spells, at +1 per 3 1/2 points of CON. Dwarves get the same bonus.
Do these stack? If you attack a halfling or dwarf with a Fireball or Lightning Bolt spell, do they get both benefits?
It doesn't seem like a CON bonus should matter against a spell that is helped or hindered by DEX.
Blodgett the halfling - DEX 18, CON 16 - has a -4 AC adjustment and a +4 vs. rod, staves, wands, and spells. Dread Delgath uses his Wand of Fire to create a Fireball and catches Blodgett in the blast area. Does Blodgett save with a +4 (DX adjustment), +4 (halfling CON-based adjustment), or +8?
I can't seem to find anything in the PHB or DMG that makes this clear, and nothing useful turned up when I searched for it. My ruling on the spot was DEX only, since I can't see why a CON-based resistance to magic helps you duck the blast of a fireball or avoid the direct stroke of a lightning bolt.
Does the Wisdom bonus vs. Will-force spells stack with this? It might . . . it's much more plausible that they'd both affect magical spells that overlap in effect. But I'd likely rule as I would above - it's either/or, although in most cases it's likely you should get the better of them. Inconsistent, perhaps, but not terribly so - and if the inconsistently bothers my players I'll just go with Wisdom affecting will-based and CON affected all other spells.
I do think that the bonus from Wisdom should stack with the CON-based bonus. One is all spells, the other will-affecting spells - it's reasonable to me that a general resistance to magic changing or affecting magic should overlap with a willpower-specific one. DEX and CON, that seems iffy. But AD&D is really brutal, and a generous approach is probably warranted to help with that. I'd like to be rules-consistent, though.
Effects of Blindness
I can't seem to find any listing for the effects of being blind. We extrapolated from the penalty for an invisible foe (-4 to hit.) It seems like it should do more, but I can't see what the rules call out as an effect in combat. Looking at the Light spell, if you cast it on someone offensively, it causes a -4 to hit, -4 to saving throws, and -4 penalty to AC. I didn't remember that during play. That might have saved a few HP had the ape's AC been lowered from AC 6 to AC 10.
Is there a canonical rule besides that for blindness?
Aside: It's highly amusing that the 2nd level Illusionist spell Blindness cannot be undone by Cure Blindness, according to the Illusionist spell's description . . . yet Cure Blindness will counter the effects of Power Word, Blind, an 8th level magic-user spell. You need to be 16th level for that. The illusionist? 3rd level. It would seem more logical if it was the other way around. The Illusionist spell lasts until the illusionist ends it or you hit it with Dispel Magic.
* I say this because, yeah, I'll take a look at OSRIC, and I'm curious if AD&D 2nd edition makes this clear, and so on . . . but I want to know if AD&D 1st edition has a canonical answer.