Precisely.
The power rests with SCOTUS.
The Supreme Court of Massachusetts is as irrelevant to the final outcome of this issue as the Supreme Court of Florida was to the final outcome of the 2000 Presidential election.
Certainly not, the SJC of Mass cited Lawrence v Texas in their opinion.
That is the first step, the second step is the "full faith and credit" clause. There are at least 5 votes to uphold that as regards the Mass SJC opinion. Absent the Mass SJC decision the case would never come before the SCOTUS. DOMA is irrelevant to a SCOTUS that cites European precedent to impose mores on America.