I have a fix for both of your problems well, almost. The preferences one is simple I experienced this myself just go to your blockland folder that is not in documents the directory is username/Library/Application Support/steam/steamapps/common/Blockland then keeping that folder up open up a new folder and go to documents and click the blockland folder. Then go to config then client, now copy the things in there and open up the steam blockland folder and go to config then client and paste them in there. It should be solved.
For the fullscreen there really is no way to go fullscreen on certain macs without it just crashing so just get a big resolution and you can check borderless if you want to.
Now if the bug is stuck and it crashes every time you open I have a fix for this too, this one is a bit more complicated but you should be able to do it, go to that steam blockland folder I was talking about and, find something I think it is in client I am not sure atm, but it is called prefs, double click it and open it with TextEdit.
Press cmd f and type in fullscreen and press enter. Once you are there turn the 1 infront of the fullscreen into 0, save it and you are done!
