They need free will to take up the Mantle, not after. I'm guessing if Molly had no intention of taking it up, it wouldn't have gone to her.
I think it is a little more complicated than that, in spite of what Mab told Harry. Molly was being prepared no doubt, but unless she was outright told this or had enough knowledge of the Fae to know what was going on, it didn't enter her with her permission. Once there I think it works like a massive shot of the most addictive drug you can think of. Lilly may have been a changeling, but at
the time the mantle entered her, she was mortal enough to have free will to reject it. The only one in the books at any rate that took up his mantle of him own free will, was Harry.. And even he didn't fully know what he bargained for once he took it up.