Can't say I've ever been to Pompey on a bike, but I might be able to help with some of the rest!
Norway: great camping, which is lucky because everything really is as expensive as you've heard!
Murmansk: I rode in from Kirkenes and back again four days later. In that time ALOT of the road that had been in terrible condition going in had been tarmaced. This happens every year because of the ruinous winters, so you may have good roads, may have hard roads.. keep yr fingers crossed.
Archangelsk means lots of terrible roads and a lot of miles. There's nothing remotely direct as far as I could see. Can't tell you anything about this.
I didn't go south through Russia but got to St Petersburg 9000 miles later via Scandinavia. Incredible city. Incredibly bad drivers. There are 'home stay' options here that will save money over hotels. Which I worked out only after I'd stayed in a hotel!
Baltic States are great. lots of similarities, lots of differences. I rode a lot of them in snow so my view of the roads will be different rfom yrs. Camp sites everywhere. Very friendly local bikers.
A tip: don't avoid Kaliningrad, though it's a tougher place to get into and around, it's an incredible, unique part of Europe.
That would be a second entry into Russia for you... I was steered away from getting a muiltiple-entry visa in favour of single-entry visas each time I needed them. Picked up the Kaliningrad visa in Riga: no trouble, but not cheap.
There are campsites in Kaliningrad but again, I was there a couple of weeks ago and camping not an option in November, for me anyway. Prices are cheap so hotels may be yr best bet.
I'd be happy to answer any specific questions here or via email. Best of luck with yr plans