BuddyPress.org

=== … relocating my .htaccess file to various locations throughout the /var/www/html hierarchy … I thought the BuddyPress installation instructions were pretty straightforward, but it seems to actually work in a whole different way from what is documented. ===

BuddyPress installation is pretty straightforward when server configuration is stable and meets standard requirements https://codex.buddypress.org/getting-started/before-installing/
which includes WordPress install already working with pretty permalinks at the very least https://codex.wordpress.org/Using_Permalinks#Using_.22Pretty.22_permalinks You should read this as it addresses the .htaccess file which is at the crux of the matter

The .htaccess file for your WordPress installation should be where WP is installed.
If WP in root, http://yoursite.com/ then .htaccess should be at http://yoursite.com/.htaccess.
if WP in subdirectory, then .htaccess should be at http://yoursite.com/subdirectory/.htaccess.

Strongly recommend you restart from scratch by deactivating BuddyPress and BP plugins, change theme to default twentyten theme, deleting .htaccess file wherever you placed it and dropping tables in DB. Reinstall WordPress and resolve pretty permalinks issues at https://wordpress.org/support/forum/how-to-and-troubleshooting before installing BuddyPress.

Good luck.