Yes, searching for "Letters" is not great. The irony is that if the app was ranked highly it would show up much higher in the results, so it's a double-edge sword.

Regarding "undoing" a letter selection, you can't, and that was intentional. It simplifies the UI (you can however reset the whole word by trying to submit an invalid word), and even if you could undo, the time you would spend doing it would pretty much wreck your score for that round, so overall it's no good. I know this frustrates some people, but after several weeks of playtesting and hearing this complaint, after the first few rounds that complaint quickly vanished, so I stuck with my decision.