Is there any easy way to unglue a return roller? I can try to pull it off (brute force method), but was wondering if there's a magic solvent available.

If its plastic and glued with plastic cement, there's only the brute force method. I have heard that putting the part in the freezer for a while will make it more brittle, and easier to break off.

Depends. You can try to use Tamiya Extra Thin cement on the area, it may enter the join and soften it up enough to remove it. I've tried it, and it worked, but only for one of three things I had to remove...
Jamie

I wouldn't break it, I would cut it with a razor saw. That way the damage is kept at a minimum.

Cheerio!

Plain old Testor's liquid cement may work - use a little at a time to avoid softening the part too much . Honestly though, I"d just cut it off. You can always drill out the spindle on the wheel and the hull and replace with brass rod. It'll be stronger than the original.

I agree - I had to do that not long ago on my King Tiger.
I sawed it off between the hull and the arm and replaced the sawed off bit with a piece of sprue.

I do a lot of restoration work on armor models. I hope this helps and it has worked for me. The old trick of using Easy Off for paint removal also seems to soften up old glue. The trick I use is to start working it gently before the easy off dries completely. within the first hour. It wont come off right away but after a few sessions, the glue brittles up and comes off. The trick works and I found it by accident. 4 or 5 times are sometimes needed and there will be cases where it doesn't work , I;m sure. I recently used it to separate the delicate bogeys on an M3 Lee so I could work the edges and seams and had no breakage.