Thanks for the kind words :) I also caught that bug and there is a lot to improve on. I agree that leaderboard and some kind of stat tracking would be great to have. To answer your questions
1) I got the word DB from https://github.com/dwyl/english-words. Since the file is 479k words, when the game launches I add all of the words into a Set for O(1) lookup time. The file itself was about 5MB, so many words but sometimes I still find some that fail or some that are questionable and I cannot find a definition online.
2) Letters are weighted when spawning, less frequent letters spawn less. For special tiles, there is a chance whether the tile is drawn from your inventory or not. When you get an "E" for example, I reduce the "E" weight so that you aren't so encumbered.