About your spoiler:
The contract in part 2 with Shikza's family was signed by Viscount Dahldof, not Viscountess Dahldof. Sylvester likely didn't expect the man to not be able to control his wife. For the part 4 "contract" of bride-stealing ditter, it was supposed to be a contract (signed and everything, not valid just because it was on plant paper), but it was NOT a magic contract. If it had been a magic contract, it would have gone up in flames when signed, and everyone in the room would have instantly realized a contract had been signed.
Also, why would Ferdinand have needed a contract to make sure Myne would do as she had promised? She's just a commoner from a poor family, there is absolutely no way for her to avoid being forced to obey orders from nobles to respect what she promised. And besides, without the temple help, she's dead with the devouring. Not like she had much of a choice to respect her words or not.
Again, I repeat myself, nothing was signed between Myne or her parents, and the temple. There was no contract at all, just promises made during a verbal negotiation.
that's my bad about the part 5 (not 4) the contract was indeed not a magic contract that much is true and was my mistake. But it would still be a contract and would thus violate the agreement made for plant paper contracts. When it comes to the part 2 incident it is directly stated that they had everyone in the family sign them (at least in the LN)
The reason as to why he would do it is 3 fold.
1st Ferdinand is very much a pragmatic traditionalist he knows their culture and their laws and will follow them unless given the Aubs permission to change them. And even then he does so sparingly only when having enough groundwork done.
2nd It doesn't take a lot at all to construct a contract by noble standards. it's literally just 1 piece of wood and some ink. that's chump change to nobles especially Ferdinand
3rd and most important: it's not that Myne could oppose him it's the other nobility that could oppose him and that could be problematic. If he didn't have a contract but just a verbal agreement it would be very easy for some other noble to just take Myne make her sign a contract (not even a magical one) and then accuse Ferdinand or the temple of stealing their servants and that would be a problem for him and not for Myne. it's not Myne that's the problem that could oppose him without it. it's the other nobles who would have a vested interest in her mana capabilities. Yes it would be easy for Ferdinand or anyone else to control Myne it's a lot harder to control the other nobility without a contract though (especially for Ferdinand who at this point in time doesn't have a lot of power over other nobles). It's not for Myne that Ferdinand signs the contract it's for his own interest and to avoid the hassle of dealing with other nobles when he can easily just say here's proof.
Either way just because we aren't directly shown it doesn't mean that it didn't happen i mean we didn't get to see what Myne was eating every day but that doesn't mean that she wasn't eating. Same for a contracts we were already shown or told that they were a thing so why would they bring attention to a thing that's been shown to be a standard already?