I have come to the conclusion that this is ultimately an ontological question. Freewill and morality are connected intrinsically. If there is morality then there is freewill. If there is freewill, there is morality.
If there is no freewill, there is no morality.
If there is no morality, there is no freewill.
Morality in this case meaning "Objective Moral Values".
Since these to are linked, to answer one is to answer the other. To make the case for objective moral values is easier than to make the case for freewill. We can look at the moral extremes and determine whether or not such things ever possibly morally good to do under the 'many worlds hypothesis'. For instance, is it ever moral to rape, murder, torture and kill a toddler? For the Moral Objectivist, the answer is easy, the answer is that such an action is always immoral, irregardless of whether or not anyone thinks it moral or not. The moral relativist has to prove that such an action can be moral under some circumstance. Meaning also, that there is no victimhood, not only that a bunch of people think it's ok.
Victimhood is the tripping stone for the moral relativist. No matter how they can justify an act societally, they cannot remove the immiseration of the victim, who won't likely agree that the torture they are experiencing is good or morally right. So you will always have a voice of dissension. For moral relativism, you need unanimity, ultimately, to prove it.
So the argument would look something like:
P1) For Freewill to exist, Objective Moral Values must exist.
P2) Objective Moral Values exist.
C) Therefore, Freewill exists.