Hey there,
Welcome to Miahar. Ill try to answer your questions to the best of my knowledge.
1) No. Nuit cannot develop muscles, there is no way around this. Nuit are inhabiting dead corpses, and they rot and are dead and therefor cannot grow. Your best bet to be 'athletic' would to be very picky about the bodies you inhabit and choosing a person with an athletic lifestyle (which means >.> prolly going to have to hunt for yourself xD because those people dont typically drop dead). Nuit are always decaying. Always. That is why they have to switch bodies in the first place.
There are ways to slow down the rate of decay, such as actually practicing embalming and getting the skill up, daily baths, you might even be able to make/find a magical item to help with that (take some RP time), but it would be very hard to stop completely since it is a natural process.
To help with the 'sluggishness' or
slight delay in the nuits action-movement you could look into
Flux.
2) I would say no to this, but I am not 100% sure. Nuit are able to take extra senses of the body they possess, and since a kelvics transformation is more magic based and not directly, like, tied into the body like an extra sense is (such as the Chaktawe) I dont think it would work. I could be wrong on this though, but my gut is saying 'no.'
Hope this helps!