I already know how to solve both contracts. Keep doing lewd things with Rikka until her stone is completely red, then she'll be freed from the contract. He'll still have his contract so he still can't go all the way with Rikka. So creampie Usui after Rikka is freed and then MC will also be freed.
Caveat: This strategy is viable assuming the effects of the contract enables his swimmers have that kind of ability to negate them. So he can't just free himself and Usui first, then just creampie Rikka after he is no longer under the effect of the contract.
Let me guess, the writer is going to make up some convoluted reason this can't be done. Or some new person will inter the equation to mess everything up. I hate with they do that, then they either change the rules, or add something new to the contract that would have been really helpful to know earlier.