Yeah, plus any plans that Lara and Mab have together for Harry is suspect.. Then there is that "look" that Molly gave Lara in Peace Talks, that says something is up that she thinks is totally wrong. One could chalk it up to jealousy except Molly didn't seem to mind Harry being with Murphy.. I think she fears for Harry in this one.
That could be simple jealousy. Molly knew Harry and Murphy were attracted to each other but she was taking the long view. She didn't have to come between Harry and a friend/lover had known Harry as long as she had because Murphy would age out of the picture naturally.
Lara is a much greater threat. Aside from the obvious personal physical benefits(which Molly can't currently provide), she offers the resources of a supernatural nation. Plus while I don't think Jim will Harry develop actual love,(can't count it totally out because it would complicate Harry's life) it is a possibility. Lara won't age out of that relationship. In fact, barring the ever present violent death scenarios, Harry will be the one who dies first from age.
I also don't think Molly can sabotage the marriage directly since Mab gave her the order to do all the arrangements. Plus once they are married, wouldn't the fact that the marriage is the seal on the alliance keep anyone on the Winter side from breaking it?