I've done a DIFF between 7.5.7-r1 and 7.5.8-r2. There really has not been much changed within the admin files, nor has there been any changes in the DB structure.
You wrote that PHP 5.4 is giving you issues, but PHP 5.2 is not.
Two suggestions;
A) Do a backup of your live install and run a DIFF between it and a set of unmodified 7.5.8-r2 files.
B) Since you cannot create the exact same environment variables as BlueHost on your own sandboxed server; Have you created a fresh/clean install on your BlueHost account, next to your current install?
Such as, if your main install is in "/forums", do a fresh/clean install to "/forums.clean" -- and do the same with your database. A fresh/clean install, right beside your current install.
Perform the same member management changes you are attempting to accomplish with your live system.
Monitor your logs. If you're also having problems with your fresh/clean install with the same server settings as with your live install, it may be time to look further. ie; are other UBBT 7.5.8 owners having the same member modification problems? Are your php.ini (or even .htaccess) settings creating this undesired effect?
If this is such a widespread problem with UBBT 7.5.8-rX rel.11-29-2013 (also 7.5.7-rX ??? rel.12/12/2012) why has there not been much discussion regarding this within the past years since its release?
Are you using IE?
I've seen crazy stuff such as this, happen within IE... honestly.
