launcher quits unexpectedly

Looks Invalid because the game code was changed

All i changed was how frequent wither skeleton skulls drop.

Does not seem to crash all the time, think it is because I need to use .Jar.

We don't support modded versions here. That said, if you want to change loot tables, the correct way of doing this is a data pack, not trying to edit the JSON files within the jar itself.

The launcher crashing in this case could be considered a bug in the MCL project (for the launcher itself), since I'd expect it to either give an error message or overwrite the jar file (since it doesn't match the checksum in the version JSON; you can work around that if you create a new version JSON and remove the section specifying the hashes and downloads but data packs are much easier). I don't have any way of confirming that myself though since I don't use OSX.

It doesn't do anything.