Hi Pascal,
databases created with 7.8 cannot be opened by older versions of BaseX (the last change came with the upgrade from 7.7.3 to 7.8). I can't guarantee you that 7.7.2/7.7.3/7.8 are fully backward-compatible, but down to version 7.6 this should be the case.
Regarding the GUI freezes[1]: I never use the .jar file, yet I experience the freeze from time to time (I always start with -Xmx512). Using -Xmx32m on purpose still doesn't reproduce the issue. In fact, I wasn't able to reproduce it at all during the last days ... we'll keep an eye on that.
Cheers,
Lukas