Yes, I’ve still got all the past versions of AR that will run on this machine installed!
And they all work without interfering with each other.
So yes, you should be able to reinstall v5 again.
AR5 and 6 used/use a common support folder, on Macs it’s found here: /Users/user_name_here/Library/Application Support/Ambient Design
Meaning both apps have access to the same Content folder that's found within those supporting folders.

For me the permanently disappearing UI panels bug only happens in AR6.