As far as uniting Scandinavia and Russia, it's really quite easy, in OTL, they were culturally united until about the time of the Mongol invasions.
Oh, sure. I was just wondering about the most plausible time schedule of such an unification.
As far as Roman Naval Power, I would think it would be more Mediteranean based, towards the heartland of the empire.
I'm quite skeptical about it, in the long term. As soon as the Norse incursions become a significant trouble for Western Europe, Rome is going to react by expanding its naval power in northern Europe till it becomes more or less balanced between the Med and the Atlantic.
With a weak naval presance, and a strong Norse invasion, I could see Britain fall to them.
Well, it may happen if Norse invasion is backed by Russia's power, so it happens after the Russian-Scandinavian unification.
On a stretch, it might also happen if Norse invasion occurs in the early phase of Norse invasion, before Rome may react by boosting its naval power in the Atlantic. In the latter case, however, it somehow defies plausibility how the WRE doesn't send some legions to pull a Hastings soon after they build up their Atlantic fleet, as it is entirely within their ability to do. In a land war, Roman legions are going to kick the butt of an early Norse army any day of the week.
As it concerns your map, I think you are de-wanking the *WRE more than a bit too much. I can see it as somewhat plausible their loss of the British Isles to the Norse Russian empire, or Southeastern Europe to the Byzantines, not both. This is still an empire that can pull on the resouces of Iberia, Italy, and a fully-developed Gallia and Germania.
IMO you are partially basing your map on cultural influences that are not really there ITTL. The Latin-Greek divide did not reach so northward in ancient times, IMO there is no real cultural or strategic reason to push the WRE-ERE border any north of the Danube or west of the Drina.
Your map is also more than a bit too optimistic about Norse unchecked expansion in North America. I would leave it off the map west of Iceland, or otherwise the Romans are in hot pursuit, and just as successful, in North America, depending on which moment we pick.