You believe that Arya will return to Westeros with ser Justin and his sellswords. But is is possible that Massey won't return to Stannis at all? I feel like the way he is written suggests this. He is not very brave or loyal, but very opportunistic. I think it's more likely that he'll switch sides to Aegon and try to marry FArya, just like he tried with Val and Asha.
I disagree. If Massey wasn’t loyal to Stannis, he’d have abandoned him during or after the Blackwater, as so many did.