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Well, theoretically, what you'd need to do is somehow determine what files the Pro Apps Update actually installed. Then you would have to find older versions of those files and replace the new ones.
Even so, the files in question would be System-internal types of things, and when you start messing with that stuff, things can go wrong quickly and even an experienced user can get over their head.
Try reinstalling *just* the Logic *app*. It should take 5 minutes or less. Also try reinstalling the Pro Apps Update, or try downloading and installing an older version of the Pro Apps Update.
There may be a couple other things to try, but if nothing works, sooner or later you need to decide when to cut your losses and go for the last resort which is guaranteed to work: reinstall your whole OS and all your apps and plugins.

You could also try downloading and re-installing the prokit update. Couldn't hurt since you're looking at a re-install of some sort anyway.
I've got the Prokit 5.0 on my MacBook Pro and everything in Logic Studio Opens. I've done 2 Logic Sessions with it installed. Everything's working."]You could also try downloading and re-installing the prokit update. Couldn't hurt since you're looking at a re-install of some sort anyway.
I've got the Prokit 5.0 on my MacBook Pro and everything in Logic Studio Opens. I've done 2 Logic Sessions with it installed. Everything's working.