The hidden words are hand selected from a smaller vocabulary list to make the game is solvable without having to guess.
Then if you find a more obscure word that is valid in the dictionary it is counted as a bonus word and can further help you solve the hidden words based on it's position.
I've also used Couchsurfing for many years and had great experiences! Sadly it got paywalled during the pandemic to stay alive and the usage has plummeted. I also always thought it could be have been done better.
The harassment messages will be a challenge. I've just added a step now that first messages will be reviewed and approved by a moderator, but would have to come up with something more scalable long term.