Your ‘activity’ renaming issue: Does that happen when you rename the pages corresponding to *other* components, eg groups or members? How about the weird redirect thing?
No, I can rename other components just fine. I renamed register to joinsite and it worked.
I did the -R flag.
As for ‘how badly screwed up IS this site?’ well… It was WP 1.5 upgraded through to 2.8. Around the 2.5 era it had bbPress integration. At 2.9.2 I moved it to WPMU, THEN upgraded to WP 3.0 MultiSite. For 3.1, I realized I didn’t need multiple sites and undid it to SingleSite. Then I was wrong, redid MultiSite, upgraded to 3.2 and here we are. If your head spun around, don’t worry. It’s not just you!
So right now I’ve rebuilt it from the ground up, currently sans BuddyPress. Frankly, I’m pretty sure the issue is NOT BP, since my localhost site, with the same theme etc, worked just fine.