Z8 is pretty much Z5 with more memory available. Z6 has a bunch of other capabilities (mostly involving graphics and mouse input) that weren’t used very much and were extremely difficult to implement, so they were left out of the later versions.
I believe ZILF can compile ZIL (the language Infocom used) to Z6, if you want to try that.
The compiled version of The Abyss definitely runs under Windows Frotz, I’ve played around with it. You aren’t going to be able to recompile Z-code version 6 to any other Z-code version easily, the screen model opcodes are just too different.