Hi again, HW Tech: I solved the problem! Since you mentioned battery swelling, I looked on the internet to find out how to know if the battery was swelling even if it wasn't detectable and read "Normal." It doesn't look like it's even slightly swollen--the Macbook lies quite flat on a flat surface without wobbling--but there was a 2008 post from someone who had the same problem I did and solved it by folding a piece of copy paper and running it around and inside the edge of the trackpad, dislodging a bit of debris that was jamming the trackpad.
I found paper too soft, so I got a white 3x5 card. When I ran it inside the edge, it went smoothly on three sides but seemed to be blocked on the bottom, so I pushed harder and a few tiny speck of white came up. I went as far as I could with the 3x5, then got a very fine needle and pried out a little more. Finally, I sprayed the edges of the 3x5 with W-D40, enough to saturate the card. Then I wiped the excess off, performed the previous maneuver with the W-D40 card, and then went around the edges again with a clean, dry card. The trackpad now functions perfectly again.
I was a little apprehensive about introducing WD-40 into the picture, but so far (two hours of steady typing), there doesn't seem to be any damage to the battery. The key is probably to make sure the card is impregnated but not wet--just enough to leave a minute trace of WD-40 clinging to the edges but not enough to wet the battery!
Thanks again for your help.
