Taking these questions in order: 1) Yes.  2) Yes, but that point in history is long past. 3) I have no idea.

The District of Maine was part of Massachusetts until 1820, when Maine became the 23d state as part of the Missouri Compromise.  The new free northern state of Maine and the new slave state of Missouri (the 24th state) were approved by Congress to keep the US Senate balanced between pro and anti-slavery interests.  The people of Maine had wanted to separate from Massachusetts since the American Revolution.  Just as Bostonians had resented rule from London, Mainers resented rule from Boston.   Resentment peaked during the War of 1812, because Massachusetts failed to protect the Maine coast from British raids.  Also, Mainers felt they were not getting fair representation in the Massachusetts legislature and that Massachusetts was imposing unfair, heavy-handed taxation on them.  Sound familiar?

The upshot was that the people of the District of Maine wrote their own constitution and affected a bloodless revolution against Massachusetts by getting their statehood approved.  Maine continued to rely on Massachusetts common law, that is, the rulings of Massachusetts judges on individual cases.  So in a sense, Maine Law originally evolved from Massachusetts legal decisions (which in turn had their roots in English common law).

As to whether the laws are still “close” after nearly two centuries of separate jurisprudence, I have no idea.  I don’t practice in Maine.  If you have questions about Maine law, ask a Maine attorney.