To me it seems somewhat odd for the show to be pushing Neviril and Aaeru to become eternal priestesses. After all, it's clear that Onashia considered it a sin, and the most recent episodes have been quite strongly pushing the point that one can be a priestess, a saviour of others, even after growing up. (Rodoreamon said this to Yun. Anubituf likewise to Guragief.)
Mixed messages aside, I wonder if we actually
will learn directly where Neviril and Aaeru end up. It's quite possible that we'll simply see the effects of their actions in the future rather than the actions (or destination) themselves. (Which would be very cruel to us poor viewers...)
There aren't many plausible options so far:
1) They go to the past either to found the Plumbish state or to change the future.
2) They go to the future and... what? Perhaps create a new choir of sibyllae to free their nation?
3) They somehow become Tempus Spatium(!) (and do what?)
4) They wind up in Yuri Paradise somewhere
5) Nothing changes - their actions are, of course,
already part of history. (So where do they go?)
There clearly isn't enough screen time for anything complex to be resolved, so I think whatever happens we'll be left guessing quite a lot...