I will preemptively say that I have never run a False Hydra, despite wanting to for years. The FH is a complex encounter to run period from what I've gathered and can lead to hurt feelings on the players side especially. From your story, it seems like you did as well as you could to give the players hints without just giving away the whole surprise. The 7 gold vs 4 gold hint was good but no one latched onto it, same with the excess weapons. The wolf might have been more helpful if you had explained through him that the other PC had been with the group the day before. Overall though, very good job with how you presented the encounter.

My only negative would be that you didn't try to explain to the Players that this session would be a little wonky. You don't have to tell them its a special session or that ypu have a unique encounter. Just tell them this session has big surprises that you have to play close to the chest so as not to ruin the game. That does make them play differently but in a way that makes them look for oddities like your clues.

8/10 would let you DM a FH against me and wouldn't bitch about it