This is a difficult one for me to get into as I'm familiar withg Niggle but have only read TGD a couple of times a good few years ago. I suspect, though, that a couple of Middle English works,
Sir Orfeo &
Pearl (the later particularly) may have influenced Tolkien's concept of 'Paradise' in Niggle. As far as the Dante connection goes we know that Charles Williams was a member of the Inklings during WW2 & Williams was one of the great Dante scholars - his book The Figure of Beatrice is a major contribution to Dante studies, but Tolkien didn't get on with Williams' ideas generally, so we don't know whether Williams ideas had any effect on Tolkien. We do know that Tolkien didn't much care for Dante's theology though...