Will Mothra work in the sequel

That depends. Suspension of disbelief is important. But honestly, i dont think they will care. The same kind of audience doesnt seem to question transforming robots from outer space or superheroes with BS abillities.

Besides, Mothra is one of the most recognisable of Godzilla foes. A big chunk will already know her.

I believe she can and the team at Legendary will faithfully adapt her and the other Toho creatures for this new universe.

Of course I'm keeping my expectations aware of the worst. Could it be G98 all over again? No, I faith in Legendary.