After all my angst over Trespasser crashing, reinstalling it, and still having it crash, I remembered that the game originally crashed right at the start of my final battle with Corypheus. I went back to the save just before then (about 5 saves back), ran through the battle and end game again, then gave Trespasser another shot. It loaded this time, so I’m very, very relieved now.
I played just a wee bit into it, just long enough to flail in glee at a scene. Now that I know it loads, I can go back and polish off the few things I had finished after my original corrupted save, then start Trespasser with confidence.