Glad you like the 6th. I thought it was a good one, but it seemed generally to be received poorly. I think you're probably right about the 'sickly-sweetness' of No 3: I'd tried possibly too hard to like that one and convinced myself I must have succeeded in the end, but there's definitely something that sets up resistance, although from the choral entry on (last 10 minutes or so) I find it much more involving.
his work seems to have had little impact elsewhere, which is surprising given that some of his technical "discoveries" could easily be applied to music very different from his
I think you're right - Julian Anderson is the only non-Nordic composer I know to have used aspects of the infinity series, though he leaves out notes, which Nørgård disagrees with. I might ask him if he knows anyone else who uses it - I've a feeling there is someone. On the other hand, it's interesting that those techniques do seem to produce a 'Nordic' sound ... and it strikes me they're not totally dissimilar to some of Maxwell Davies' magic square techniques, which could be said to produce a similar sound (not that I hear that so strongly in the string quartets I've been getting to know recently, although the technique itself is still very much in evidence). I don't know any of Nørgård's quartets, actually, though there's about 15 of them IIRC.