What I think is happening is this: The middle chain ring is "picking up" the chain when the chain is on the smallest chainring and the smallest cog.

I thought it was mainly the derailler at first because the derailler does infact rub against the side of the chain when the chain is on the smallest chainring and the smallest cog, and the 2d and 3rd smallest cogs as well.

But when I push the derailler out with my finger a tad bit (is this harmful, by the way?) so the chain does not rub, I then spin the wheel. If I have someone apply a resistance to the wheel when I am peddling, the middle chain ring will pick up the chain just a bit and then drop it back down. Usually it doesnt completely send the chain off the ring but this did nearly happen when I was riding the bike.

All of my 'testing' is done with the bike upside down since I don't have a stand. Is this ok to do?

Before this whole mishap, and after the bike shop had adjusted my front derailler, my gears were working like a charm. Although the front derailler did sometimes rub the chain on the side, the front gears changed awesomly and the rear gears changed even better.

I was even able to use the chain on the smallest 3 cogs and the smallest chainring and I didn't have these kind of problems at all.
What I am thinking is they replaced the chainrings and they were slightly different. I remember him saying about the middle one, as he picked it up "Oh, this is an LX" which was apparently SLIGHTLY different than mine. I asked him how it was different and he thought for a couple seconds and said "they're pretty much the same thing" so I was like ok...........

I am pretty sure I didn't mess up the gears when my chain slipped and I bent the chainring - do you think this could be a possibility though? I can't really see how it could have bent the front derailler or anything, when it slipped...

The bike really doesnt have too much use on it so I wouldn't think that the chain is THAT worn...

I'm not familiar with a 12 inch point and measuring a chain. Can I just do this with a normal ruler? And, what do I do?