Thanks for the feedback. These are both interesting issues - to address them individually;
1. They are all real, actual words...English is a big language. However whether they are reasonable is a more difficult question and one not easily sorted by just reducing word length. Overall however the strategic player still has a lot of opportunity to cut off a word at source by thinking ahead.
2. This is what is known in games as zugswang and it can seem a bit cheap. However, there are lots of ways to extend a word by one letter other than 's', for instance by adding 'y', so this is difficult to judge without breaking inner consistency. personally I think that avoiding giving the computer a plural is just part of the game, like not playing a passive game in chess and allowing a pawn to reach the end of the board.