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.
Oh, it is certainly not a real solution—I doubt whether there is any. But it helps to some degree, in many cases.
The classical way to deal with this problem is critical thinking: whenever you seem to arrive at a certain conclusion, do your utmost to defend the opposite conclusion (or some other proposition entirely). If this is at all possible, you must admit that you simply do not know the answer (yet).
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.