I agree they wouldn’t (force custody) but maybe it would be used to say they have to agree an amicable joint custody arrangement between them or else she (or Charles) will intervene and take control
Thing is, if issuing ultimatums you have to make sure they can stick; whether it would or not I've no idea, but I imagine Meghan's lawyers will have a view
Having thought about this, I’m sure that (despite the parents being divorced) year after year the Queen has been with her grandchildren at important events (eg Christmas time etc) and I wonder whether she has enforced this. Diana and Fergie definitely said at one time that they missed seeing their children at Christmas and it seems odd that they didn’t do alternate years. I would say that at very worst, it at least guarantees her children joint custody.
Also, I don’t believe you can take the Queen to court so I’m not sure what grounds Meghan’s lawyers would have anything to say (although undoubtably they would try as they would want the money).
Of course, it’s possible that H&M will remain happily married. If so, and they continue to refrain from returning to England, perhaps when Charles is monarch he will use the law to enforce time whey Archie. 🤷🏻♀️