Is it haram to cheat and if so what is the punishment?
If there was a condition in the Muttah contract that the male cannot take another partner and he breaks the contract and does, what’s the ruling/punishment?
As well if he forced the girl to change her mahr and kept prolonging giving her it?

Yes cheating is not permissible and it’s a sin.
If the contract is broken then the wife has the right to annul the contact. Forcing change of mehr is not permissible