How do I get leave of court to file an opposition to my motion (rule 60b)

You can write a letter to the judge, requesting leave. Make sure you send a copy to all other parties. In the Superior Court, you have to comply with Superior Court Rule 9A; see below link. In your letter, briefly explain why you feel compelled to reply. Be aware you will (probably) have a chance to make an oral argument. In probate court or district court, Rule 9A doesn't apply, but there may be other court rules which do. If this is an important matter, you'll do better with a lawyer -- sounds like a fairly procedurally complex issue from here.

I think that more information is needed. You should consult a lawyer, even if just to flush out all the facts and other relevant information. A motion to reconsider has a separate rule in the Probate and Family court that must be followed. But, a lawyer will need to know exactly what it is you want reconsidered, so you wold want to bring the whole file you have on it. The Middlesex Cty has a lawyer for a day program just across the river. Go see one and bring your file.