For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
I really don't know. I think it should be no problem to install the plugins you need, but as long as the reason of this strange display isn't known...Do you think it's 'safe' just to do that for all of them or might I cause other problems?
Does this change backend URLs, too (I've never used it)?sh404sef