I am not all that techy and I can't say that I know every detail of Provably Fair system. I know basics and understand how this system works.
However there were cases in the past that users found a way to tinker with Provably Fair system and manipulate outcome of a roll (Hufflepuff incident).
And now my question: can this scenario be reversed somehow? So that House could in the theory manipulate the rolls?
Hufflepuff incident wasn't manipulating, he basically had an second account with same server secret, so he first bet there and with knowledge of outcome (or with revealed server secret from second account) placed bigger bets in his first account.
If everything done correctly, manipulation is impossible, operator cannot change predefined result. Without client seed, or with client seed generated by operator (even locally, as operator can send it to the server before generating server seed, very hard to catch), operator can cheat by learning players' behavior. Example: You always bet High 50, so by knowing client seed, operator can generate server seed that give more Low 50 results (but this is too dangerous for operator because you may decide to play Low 50 this time).
Provably fair is a best thing happened to gambling, after bitcoin