This isn't a reply to anyone in particular, just an observation/bucket of cold water. Seems to me the NIP has 2 fundamental problems, firstly, the electoral system. Labour is half way to Pasokification, but is being held up by first past the post, which leaves it as still the only alternative government. It would have to be with SNP support and it isn't a very likely government at the moment, but still it sits there, occupying a space. The other is that NIP is no Syriza. As far as I can tell it hasn't emerged out of northern working class communities, doesn't have union backing and isn't likely to become a 'movement'. It feels like a 'project' for a small number of activists - nothing wrong in that, given that many small parties start that way, but not really a formula for growing deep roots.
On the ''northern socialism' thing: is that a 'thing' or are we talking about a social democracy that was specifically labourist, municipal and union based? There may have been more of that in the north and at times it 'felt' northern, say in the resistance to Thatcher. Maybe it was eroded earlier in working class communities in the south, but is there anything uniquely northern about the thing itself? Ultimately, the issue is about building resistance in working class communities, workplaces and elsewhere.