i think Bardoon & FSLAR may have it right, that if an immortal being fuses with a mortal being, it negates the "immortality" status. who knows, this whole plot has been wacky as hell. Although, if i may say another theory (that i've gotten from Dr. Strange, spoioerz), perhaps Zamasu gets caught in a time loop. Since he's immortal & can't be killed or sealed, the only other option is manipulating time yet again so he can't escape the reality he's trapped in, via the Time Rings. it would kinda be a "tragic irony/poetic justice" ending, since he sees himself as punishing Trunks for manipulating time, yet he also receives dire consequences for his actions despite being a "god." imo that would actually be a better conclusion, something we haven't seen before in Dragon Ball, as opposed to "ki blast em till they're dead" approach we always get.
prolly not gonna happen, but i had to throw that possibility out there lol