Changing the value for NumPlayers shouldn't cause any problems with the game or saves. It may need to be re-done if a future update changes that file, and a backup copy of the file left in that folder can cause the game to crash on startup (as noted in the link above), but lots of people have played with the either the 4 player mods or the hex edit.
The editor is being updated, and there are usability improvements planned. The same engine is being used for Larian's next games, so development of the editor will continue.
