Have your lawyer offer BM an out of state visitation plan with you keep custody. You can offer a half way point, every other weekend (if she doesnt work than maybe a 3 day weekends) but that child should return and remain in same pre-school so she is not disrupted again.
If this goes to trial is in on BM why she can move (yes have your stuff ready to counter anything she may come up with) but the child was already removed from her care. Just show she is doing well and adjusted. Ask that any future courts dates, your family court will have jurisdiction in this case.
What is the schedule now? How much visitation is she getting now?
I would not sign anything that says you will re-visit at age 5. You are paying now and in the system. Get is all covered now....detailed parenting plan with times, dates, vacations, summers.