If they change the hashed seed, than hashing it again to verify will return a different result than what the server gave you, so you know they cheated.
Most verifiers check to make sure that the legit hash was used.
How do the verifiers check the legit/original hash was used?
The hash is provided by the gambling service. They can just issue you a fake hash and provide you with a fake solution and you will never know right?
You have a point. I don't have technical analysis to share but for a thousands of professional online gamblers in the world including in the crypto world, Im pretty sure that it should, at least one of them can technically and professionally verified if a certain site has unusual shit doing around their verifier.
Also for a famous gambling site, at least in some majority of the cases, they will not ruin their reputation for that rigged hashes or something as running their operation continously will give them huge earnings than to shut down all the things and get all the money from the players.