My sorcerer's HP permanently dropped to 33, while everyone else's is in the hundreds, and no, it wasn't always like that, as she could hold herself in battle enough, but now she dies within the first three seconds of battle each time, even my party members get weak just from walking too much, so she sometimes ends up dying just by walking and not checking up on her.

Her age is 22. I feel if this was due to her age, then a glitch makes the game think she's 90.

Also... I should question that my characters all get weak massively fast on the world map.

Makes me paranoid that I'm holding a cursed item.

Edit: Also, my Cleric is age 22 as well, in fact all my party members are, despite my Cleric was 34 before. (For some reason he aged faster than the others) maybe my game, particularly the aging is glitched?

Max HP depends from 2 factors:
1. Level.
2. Endurance (End).
You most likely have something happened with your Endurance.

Perhaps, but I checked everything she was equipped with, and she isn't holding anything that alters Endurance, and I have everyone completely healed as much as I possibly could, and it says she has no status conditions. In fact she's holding a troll ring, which gives 20 HP.

Just noticed... her endurance is 1.

Makes me wonder if the fact I was massively spamming "Wizards Eye" did something to her.

Was that character poisoned/diseased, went insane or something?
Wizard's Eye should have nothing to do with that.

As I said, I cured her of everything, and it says her health is fine.

You need to put a screenshot of status screen of defected character. Hard to judge without it.

Oh my... that is quite a hideous character to the left of defected one.

It might be that you used Raise Dead on your sorcerer too much, it lowers your Endurance by 1 each time you use it.

That MUST be it, as I did. Being a "Squishy Wizard" as TvTrope puts it, she wasn't that high defense to begin with, and I entered the Tower of Magic possibly way too early as everything there was massively strong, but then again I had access so I assumed the time was right.

Is there a reverse? Or should I just start over from scratch knowing my mistakes?

There will be Yak Temple where you will be able to raise your Endurance up to 21 if it is less than that. Personally, I always reload when any of my characters die, it saves from a lot of bad consequences.

I assumed, tbh that the wells and such that raised stats were only temporary.

And thank you :D

There are many permanent stat boosters in World of Xeen, so keep an eye on them.

As was mentioned before, one of the wells in the Temple of Yak will permanently give +5 Endurance to any character with 20 or less base Endurance (not counting modifiers from things like age or equipment). This can be used repeatedly, but I believe it also inflicts a (curable) status ailment at the same time. If a character has 21 Endurance, you can even deliberately kill the character, cast Raise Dead (permanently lowering Endurance to 20), and then get another +5 bonus. (A similar trick works in MM2 as well.)

Why doesn't the manual mention the permanent side effects of certain spells? (Resurrection causes aging and also lowers Endurance. Divine Intervention ages the caster 5 years (reversible with a repeatable quest or a fountain) but doesn't lower the Endurance of those revived.)

In any case, this issue is one reason I don't like it when resurrection effects have permanent side effects.

I have a vague memory that using a temple to raise a character from the dead will do so without the Endurance penalty. But I could very well be wrong about this.

Also, the permanent side effects of these spells are definitely mentioned in the manuals for the earlier games (MM1 and MM2). Are they not mentioned in the Xeen manuals?

They are not mentioned in the World of Xeen manual (don't know about the Clouds (alone) manual), though I believe they are mentioned in the MM3 manual.

Of note, the MM2 manual doesn't always match up to the game. In particular, in the DOS version:
* Holy Word doesn't age the caster.
* Frenzy's Endurance loss is temporary; it goes away at the end of the battle, and it doesn't affect HP gains anyway.

Edit: Another thing about MM2: Uncurse Item sets the number of charges on an item to 1 (even if it's 0). It doesn't actually remove a curse on an item, even if you somehow manage to get such an item (must be +63 and Neutral only, I believe).