Thanks for all the suggestions, comments and bug reports.
When I have time I can add an end screen, fix the Opt1 bug, let the music loop.
For difficulty fix you can just add more trees by pressing <PAUSE>. After adding 32 more trees you are guaranteed to have just one solution.
For me the current difficulty is just right. Just put cottages on the board and move them around. You don't need a single solution. Just find any solution.