Based on the many excellent suggestions on this thread, I processed another roll at room temperature, which today happened to be 22°C. This roll came out much better I believe, and I was even able to get somewhat usable images out of two *very* overexposed frames where I was experimenting with long exposures. There are still weird black squiggles and lots of dust spots, but I'm pretty sure these are due to dust in the camera when the pictures were taken. I'm working on a roll of Delta 100 in my other camera which has given me flawless color pictures in the past, so hopefully that roll will be less blemished. Here's a sample:
A word about my scanning process, as mentioned above I'm using an Epson Workforce 500 flatbed at 1200 DPI (the max it will do), placing the negatives on the bed, covering them with semi-gloss freezer paper, and then placing a iPad displaying all white on top of this. The results have been usable, as I'm only trying to digitize these photos to share with friends online. I'll make prints the old-fashioned way sometime in the future.