In only one VI campaign did I "cash in" my monasteries before the Vikings could get their hands on them. It's something of an exploit, but with the Scots there is not much else you can do.

With the Mercians though, you shouldn't have a problem. Lindissi is the only Mercian coastal province, and it only has a Forest Clearing (indestructible) in it. Viking raids are a worry for the other poor saps on the map, not you. You need to focus on eliminating the Saxons before they get big. The Mercians also get Urban Militia/Militia Sergeants for AP troops, so when the Vikings do come around you have something to hurt them with. You should be able to crank out UMs right from the start. And to add to the embarrassment of riches, Mercia gets Mounted Sergeants. Great against the non-Germanic factions.

I would say attack the Saxons and Welsh first, focusing on their landlocked provinces first. Build out from there, while the Vikings do their raids and weaken the other factions.