In the early part of his career, William Marshal is simply landless knight, a younger son albeit a phenomenally talented one high in royal favour (especially of Eleanor of Aquitaine). At 42, on his death bed Henry II promises William the hand and the estates of one of England's greatest heiresses, the Countess of Striguil, Isabel de Clare. Henry II makes the promise, but Richard Lionheart confirms it. At this point, William has the estates (though not the Dower properties as his wife's mother is still alive) but is not given the title. He gets the title, Earl of Pembroke about 10 years or so later, I believe by John. After being given the estates and hand of Isabel, overnight William goes from being a landless knight to one of the richest men in the kingdom and has the status to become a commander and eventually a statesmen. The process certainly doesn't happen overnight. While William Marshal was an entirely appropriate choice for regent on behalf of Henry III at age 70 (he was fit and sound of mind and even led armies at that age!), I don't think he would be appropriate choice at 42, after he had just come into his estates and had little to no experience governing.
I know he wasn't Earl [of Pembroke] yet (call that a missed anachronism) but he was Marshal of England and would thus be responsible for the security of young King William III. A place on the Regency Council would be expected for him.
This also then raises the odds of Margaret being on the Council assuming she is still in the country.
I assume he'll still get his marriage finalised if not the title - that may have to wait until William is of age raising the possibilites of other titles instead.
Hmmm, it looks like a broader council will therefore exist - who else would be on it?