Progress! After failing rebuild-search-index countless times, I downloaded CA 2.05 and ran rebuild-search-index and a couple of other cautils before that, and finally, the re-indexing went right through. Now, if I want to add a related object to an entity, for instance, typing in the name of the entity brings up objects that also have that name.
This has at least given me a "back door" solution to the "upload media" problem which still isn't resolved. Because, as I discovered 2-3 weeks ago, I can upload photos to "photos and images" - one of our categories of Objects, so long as the image (filename?) includes the name of the entity, it finds it, and when I view the entity in Pawtucket, it shows the photo.
Another issue, likely related, is that if I open an existing entity in CA - one that was created under 1.7 with an image uploaded, the thumbnail at the top of the "Editing Individual" sidebar is not currently displaying - just a small icon at top left of the box where it used to appear. Clicking that icon successfully opens the image, but not within the thumbnail box. Some reading of past CA support threads suggests there may be a problem with tilepic?
Anyway, I am reassured that our database isn't corrupted, which I was starting to think when it was failing rebuild-search-index.