This is a tough one, because one of the optional mission objectives for the second stage of the game is to hold 2 provinces in Britannia. I've found that re-colonizing them (obviously) takes longer, so deconstruction rather than abandon might be a better route. I did try this in a previous game and what ended up happening was the Celt factions would sometimes conquer the settlements, sack, or raze, but the Ebdanians would continue to sailed raiding parties to Spain. It didn't really make a difference for me "foreign policy-wise" to keep or abandon Britannia settlements. However, it's more of a pain to have to re-colonize and probably more expensive too. There was one campaign where I did not abandon the settlements, and one of the settlements was taken by WRE separatists--I had repelled a Caledonian siege the turn prior, and the settlement was easy picking in the turn after. I could not end the civil war until much later in the stage of the campaign, and this was what made me decide that it might be better to abandon. The Danes have been consistently passive in all of my campaigns, and the other Viking factions never invaded me in the first stage of the game. I guess that might change on the legendary setting though.
I've tried this, but the Franks always try to bill thousands of gold from me to join wars (although I did not try to do this in the first 3 turns of the campaign, when almost everything seems to cost less). If I give each of them a small gift, I can usually keep them at peace with me unless they happen to be allied with another faction I go to war with (occasionally happens). I didn't care much about maintaining peace with the African factions--their units are soft and munchy for my generals Although it's probably not a good idea to be at war with all of them at once, but I generally just need one stack to take care of that region.The big ones are probably (a) getting the Franks to fight the Saxons; (b) getting the north Africans on-side. It depends partly on their personalities, but often with a little cash, you can get the Franks and Saxons to fight to the death; and get a North African as a reliable friend. I don't pay more than 700 gold ish for one deal. You may also be able to postpone some Viking forefathers' assaults.
Yeah, I've also declined to go to war with the Alans when ERE requests. Unlike the eastern front factions, the Alans can migrate and roam the map. The Visigoths, on the other hand, is an optional mission objective for the first stage of the campaign, so I always try to get as much gold from ERE to delcare war on the Visigoths, since I will do it myself anyway. But they can do a lot of damage in concert with the Ostrogoths.One thing I learnt recently was not to heed ERE requests to declare on the Visigoths and Alans. They promptly switched their focus to me, doing quite a bit of damage.