This comment has put me off for a bit:
# ∆ Remove this line after writing the letterInWord function.
Took me a little while to notice that this was just for debugging purposes, but in any case, it looks like removing the mentioned line is mostly an optimization and it is still possible to win without removing it.
I’ve also noticed that Python is missing String instance methods, but alas, this is unrelated to this level. I’ll report this in the GitHub repository.