1) Most halmaheras are wild caught as they are apparently difficult to breed. There are some CBB, but I don't know the breeders.
2) Not sure about temperament, but humidity requirements are very different. Northerns are very commonly captive bred and require about 40-50% humidity where Halmaheras require around 80%.
3) Not sure about a CBB. Due to their difficultly in breeding, I doubt there are many axanthic CBB. I don't believe the WC are super expensive, but then of course you have to deal with vet bills due to the possibility of parasites. I'm sure a WC axanthic would fetch more, but not sure how much. I thought I've seen some on Ben Siegel's auction, but I can't recall how much they went for.
4) I currently have my Northern in a T8, but I am looking into getting a taller cage. I also prefer my lighting mounted inside the enclosure rather than have a cutout. I'd think a T8 would be fine if you have a hole cut in the top, but most people recommend the T10 with the hole cut and a T11 or T12 if mounting inside.