looks like a great trip, though it involves covering a couple of the distances twice (youd be spending some time on the train, but the scenery's nice).
as for the seat reservations it doesnt look like its possible to buy them online. but you need a seat reservation on almost every long-distance train in norway (the oslo-gothenburg being the exception i think).
if you live in denmark now you could probably just contact DSB (http://www.dsb.dk) at a station or by phone or email and they'll probably be able to reserve journeys in norway.
alternatively you could contact NSB
NSB Contact page in Norway by e-mail or phone.
you might want to know that norwegian night trains dont offer many alternative ways of sleeping. i think, but check with the nsb or dsb, you either reserve a twin coupe all for yourself (which isnt cheap!), or you reserve a seat. sleeping in a reclining seat can be ok for someone and you get free blankets and eyepatches and stuff. there's no sharing coupes like in the rest of europe unless that scanrail pass somehow mixes people together (doubt it).
my tip is therefore to avoid only taking the night trains and rather enjoy the nature (especially on the oslo-bergen journey) along the tracks. hope this was helpful...
good luck