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 can't see the relation to your 2nd post.mysite.php?variable1=var1
You've asked
I can't see the relation to your 2nd post.
No idea what you and Jumi are doing there (using variables $xxx outside php code)