I have noticed some odd behavior with the Willpower stat (low Willpower seems to protect you better than high Willpower in some cases, and for Sorcerers high Willpower makes Spell Reflection less effective), and therefore have come up with the problem of how to lower the stat on purpose.

I have already tried hitting myself with a Drain Willpower (area at range) spell; unfortunately, doing that crashed the game. (Isn't it annoying when that happens?)

I *could* use curse, but the problem is that that lowers other stats as well, and I don't want that.

Any ideas?

I guess you have tried different variations of the Drain spell already, and confirmed that the game crashes reliably when you use it. And I don't think there are monsters or events which drain attributes in Arena.
If I remember correctly, there is a Transfer spell, but it works the wrong way (target -> caster).
You could hex-edit the attribute field if you can find it, I guess.

Well, stat draining *does* occur as a result of Disease or Curse, with the most well-behaved case being Disease inflicted by an enemy's attack.

I was able to get Willpower damage by using a Curse spell, however, it also damaged Intelligence, Personality, and Luck. I think I may try combining it with a Fortify effect to see if I can trick the game into thinking that my un-fortified stat is lower than it would normally be, and then rest to restore my other stats.

Curse spells don't behave as you would expect. In particular:
1. The spell only damages your stats once. You become immune to further curse effects until the current one wears off. (The Cure Curse does not appear to work properly, so it doesn't appear possible to end it prematurely.)
2. The amount of stat damage is equal to the value of the Y value of "X rounds per Y levels", so if I set the duration to 1 round per 20 levels, the affect stats decrease by 20. (Note that this parameter does not affect the duration as it should; however, increasing it *does* reduce the cost.)

It seems the programmers did not actually test some of the more obscure and less useful effects as they should have. (This isn't the only bugged effect: Cause Disease seems to either do minor Intelligence damage over time or kill you in 5 seconds, for example, while Continuous Damage is weaker than it should be but lasts forever.)

Edit: My idea about using a Fortify effect worked. So, the trick (for lowering stats affected by curse) is as follows:
1. Inflict yourself with a Curse. The duration should be set to 1 round per 20 levels (which actually gives it a magnitude of 20).
2. After your stats drop (you will notice your magic meter get fuller when this happens), use a Fortify Willpower (or whichever stat you want to lower) spell. The magnitude can be as low as 1, but the duration should be long enough to rest to heal the stat damage.
3. Rest until your stat damage is healed. The Fortify spell should be in effect.
4. Wait for the Fortify effect to wear off. Fast Travel will work for this purpose.
5. Your stat has now been lowered, and will remain at the lower value until you rest.

So yes, it *is* possible, but is more cumbersome than it should have been, and is definitely not how things were intended to work.

:-)
I have a lot of experience with programming and with managing development teams (for business systems), and I think it's fair to say that you expect from the developers and testers too much or the wrong thing - no offense intended.
Arena was supposed to create a new game world, a pilot game or proof-of-concept, as we would call it today. I don't
think the creators of this world ever anticipated the degree of success they would have with the TES series and in which direction it would develop.
They created a new role-playing system, which gave the players a very high degree of freedom and flexibility (still rudimentary for Arena, but evolving with every new TES game), and I don't believe for a second that they bothered to think through every aspect and implication of their system. Also, testing the game to the level of detail you expect (and I don't disagree with you - "in principle") would have been a time and cost nightmare (just consider the effort to plan all kinds of tests, to prepare the test cases, and to do regression testing for such a complex system with lots of interdependencies). Like you say, they very likely didn't really test the functionality they found not so important, or too difficult to test.
As a general rule, which applies to games as well as to business software, an experienced user understands a system much better than the developers. And users can do a lot more with the systems than the developers ever thought of (this includes database software like Oracle, financial systems like SAP, etc, too). In my active time, we always went back to our users and listened to them, not only what they liked and didn't like, but also what they did with the system, in which way they explored the boundaries of the functionality and in which way they managed to break it - you wouldn't believe how creative these guys were!
So don't be surprised that you are one of the most knowledgable people when it comes to understanding the innards of the TES games, and to have an understanding of its functionality and mechanics which exceeds that of the developers by far.
You are right to criticise the shortcomings of the TES games (which there are plenty of), but we should also accept that these games give us something which has no parallel in the gaming world: a role-playing system and environment which doesn't simply tell an exciting story, but has given us a world to live in, and which keeps us interested and motivated over decades. Is there another game or game series where the developers gave the users anything comparable with the TES Construction Sets?
My personal opinion is that the latter exceeds the design, programming and testing deficiencies and inconsistencies, and the occasional instabilities, crashes and save game corruptions by far.