I vote for salvator, though I am not an expert in Mediaeval Latin. In classical Latin, at least, the Greek would be tacky. Both words sound rather Christian, but soter even more so than salvator.
I think modo is an improvement over solum.
Nihilitas sounds much weaker than nihil: I’d prefer the latter. We shouldn’t think English: the -ness part doesn’t need to be carried over. Then again, it is possible that nihilitas was a favourite word of 13th-century literature.
I think I agree that quidquid cannot normally be split up; but is that reason enough to say it must be one word? The particle -que cannot normally be split up either, but it is split up occasionally in poetry, if I remember correctly. I think what constitutes a word and what doesn’t is ultimately an unreclaimable quagmire, though in this case I’d certainly prefer quidquid over quid quid too.