I would be tempted to go with just "Nullus soter soteri", a more poetic construction that in English would be closer to "no saviour for the saviour", leaving verb 'est / there is' as implicit. This would also line up nicely with the following lines.
edit: also "salvator" seems a better match to the "rescuer" meaning of "saviour" than "soter".
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.
If anyone can do non-wrong Latin, I could use a translation of the following for HPMOR. The original is supposed to be circa 1200.
No rescuer hath the rescuer.
No Lord hath the champion,
no mother and no father,
only nothingness above.