You made me look at the Smyril timetables again; it's like a mental torture.
It looks to me that they have changed the route and sailing times for 2008. Only go to Scrabster late June July and August. I can't see how you get back from Iceland without a 3 night stopover in Faeroes and then back to Scrabster via Norway. If that’s the case see if there is an alternative route back from the Feroes, maybe via the Shetlands. The only other alternative way back with Smyril, Iceland, Faeroes, Denmark, (approx 38 hours) and then catch a ferry back to the UK from there.
There is a sailing every week in to Iceland so your minimum time to stay would be a week
The main roads have sections of dirt, the surface is very good and should not present a problem. If you venture into the interior then things get much harder. You will find some road info here,
Driving in Iceland
and if you do a google you'll find more.
I am a Guzzi fan, so like Walkabout asked, lets as know what you think of the Norge.