When the player betrays egil, and joins the saxons, there is no penalty, egil just vanishes, so does the viking lady, no penalty, no impact, they are gone from the story.
as far as reality, and the possibility of possible stories.... How realistic is it, that you become the lapdog of a Jarl, by just showing up at his door ? Absolutely None
Better yet, you and a bunch of farmers you recruited, are able to hunt down, and kill this rebel leader, that the jarl, having hundreds of men, was not able too
And as far as Thiaderd only having a few dozen men, because that is what we counted in the battle more or less, then when the vikings invade the village, they din't have more than 50 men.... How where 50 vikings gonna help the jarl overthrow the king ? This game is not realistic in the least, i can draft 50 men from a village in a week, that is not realistic.
Also the idea is not to replace, is too offer a different path in a linear story, the story is on rails, from start to finish, it only offers an illusion of choice, there is no penalty other than the loss of numerical value in stats, or having one of the horribly developed characters leave. Which really is sad, there should be way more interaction between the main character and it's party, why not have them show up in your camp/refuge/castle/mead hall, where you can interact with them, and ask them to shed some light into their characters ?
And as far as it being hard... Being that the story is the only unique thing about the DLC ( Vanilla offers sandbox, you can play vikings with mods, Multiplayer is not new) they should go the extra mile, and many games have had different branches and approach scenarios for their quests: Baldur Gates, Never winter, Fall out, Fall out 2, Wasteland, wasteland 2, Shadowrun returns etc etc. Is nothing ground Braking.