Is Nanachi able to ascend from the 6th layer again safely due to being a narehate? I'm not sure if it's been covered but because Nanachi already has the blessing does that mean that as a narehate they can ascend without dying in a similar fashion to Mitty?

Wiki:
http://madeinabyss.wikia.com/wiki/Nanachi
Nanachi is able to see Curse, so she is able to avoid it. You can also remember that she've established her hideout in zone free from Curse, which may mean she still needs to avoid it.

So, can she ascend from 6th layer again? Probably yes, if she can find zones free from Curse. Is she immune to Curse? Most probably - no, at least there were no apparent facts proving opposite.