Military service member's state of legal residency.

[Opfor77]

My mind is fried right now, so I apologize if the answer is simple.  Here are the 2 questions:

1) If a servicemember lives on base in North Carolina, but has their state of legal residence in Massachusetts (per The Servicemembers Civil Relief Act), are they considered a part-year resident of Massachusetts, or a full year resident?

2)  Since the nonmilitary spouse of the servicemember can also use Massachusetts as her state of legal residence, even if she's never lived there (per The Military Spouses Residency Relief Act), she can use Massachusetts as the state she pays taxes in for her own W-2.  What if she didn't have Massachusetts chosen as her state of legal residence yet on her W-2 in 2019, or 2020?  Is that option of Massachusetts state taxes gone, or does she have to go back and file an amended 2019 return, choosing to pay Massachusetts state taxes instead of North Carolina, and filing both a 2020 Massachusetts state return, and a North Carolina return to get the refund?

I apologize if the question isn't clear, or if my mind just isn't firing on all cylinders today.  Thanks!

[Marsha2020]

The service member would be a full year Massachusetts resident. As for the wife, it might be more advantageous monetarily to file a North Carolina resident return. As for 2019, I would not amend. If wife is working in NC, I believe it might also be difficult to have payroll withhold Massachusetts taxes. Just my thoughts. 

[Opfor77]

Thanks for your input, I appreciate it.