Okay. Sorry about the delayed response. I did go ahead with some (all?) of the tests recommended here, but never checked back in. It's just been a crazy couple of weeks for me.
Before I get into things - Olympus. So the camera listing is for an "Olympus Stylus Camera"...I'm guessing this is the original Mju, then, right? "You hear the film advance" how loud is it? I'm figuring for $30, it might be nice just to try out for fun. Plus it can focus through windows, which the Hexar can't (well it can, but it's a guessing game...sometimes yeah, sometimes no). Yeah, I like 35mm focal length. The XA/2 I've looked at previously, but I know the XA at least is a rangefinder, which I'm not in the market for right now. If something popped up locally, I might snatch it, but I'm not going to go out of my way for one like I did the Hexar.
Ok, with that out of the way...
I did the test with the camera back opened, trying different lighting to see if the aperture adjusted properly. Seemed ok. But I may have missed something with that, I'll re-read that tip. I also looked close (initially with magnifying glass, but using my own eyes worked better) to see any oil on the aperture blades - nothing. The only thing was some scratch marks, presumably from the blades sliding across each other for years. I did an on-film test of a low f stop and a high f stop to see if the camera compensated correctly. But I haven't been able to process that film yet, I'm hoping to do that tonight, if not, tomorrow. I'll post the results.
With that, my only thought is what someone earlier said - I could've just blocked the meter reader on the front of the camera. I checked how I normally hold the camera, and I wouldn't block it. But it isn't a stretch to assume a finger moved in front of it without my noticing whilst shooting.
That's a good point about the exposure comp dial. It probably never was really bothered with. I can definitely move the dial around to loosen it up and make sure the contacts are working. What is meant by "electrical contact cleaner" and having the switch (what switch) opened?