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.