A nice feature would be "locking" black squares, or otherwise marking them somehow, to distinguish them from squares you may still switch in the future.
Agreed. The black squares should turn white once a row AND column are correct. Otherwise, you just spend you time scanning right and down checking if that row/col is right again. It is tedium and not enjoyable.
Yeah, you should be able to manually lock or they should lock automatically once it’s guaranteed a tile is in the right place. Like if there’s only one black tile in a row and that row is correct.