Do you know how far away their house is? There might be more to it than they've said, twice a week doesn't sound excessive. My brother's girlfriend used to stay at ours for weeks and it seemed fine. Maybe your daughter enjoys cooking, I sure do, no matter who I'm cooking for.
Maybe you need to give them trust in their relationship and give them some space. She is 26 and probably understands. Saying things like their relationship might not last might make her uneasy as if you don't approve which would sadden her, be happy she has someone.
I understand maybe you won't want them doing stuff in your house, but she's 26! Even if it's your religion it might not be hers, don't stop her being who she wants to be and do. She's clearly trying to be independent since she's cooking and buying groceries and working. Be proud of her. Everyone gets sad when they break up it's inevitable, but let her enjoy it while she has it.