Ack -
6.5 is BETA
So you really shouldn't run on a live community unless you're able to fix/change stuff yourself.
It's always near impossible to go backwards a version - as the database structure changes with each version. Particularly a version like this where vitually everything changed.
If you wanted to go back, you'd need to restore a 6.4 copy of your database (which doesn't have any posts made after you upgraded) then restore 6.4x files.
I believe it's been posted the arrow thing with new posts will return to normal functionality in beta5. There's a mod at threadsdev to give moderators access to the control panel to edit their forums I think.
Tis the trouble with beta. I know this won't help you much now. But betas are usually unstable, so unless you can fix/change things yourself in between versions to avoid problems, you should setup a test forum to test the beta and bang on it hard before rolling it out to a live forum. Particularly a beta version where so much stuff was totally rewritten (the control panel).
Best bet is to sit tight, now that you're on 6.5 and keep upgrading each time a new beta comes out, then the bugs will slowly go away. <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />