The card is already lore friendly (I mean shakles, lock a unit...I don't know how it can be more friendly) but the idea is interesting for sure.
The only issue I have with it is that, either you make it not able to lock in which case the card will be useless (but I'm pretty sure it's not what you meant) or it locks the unit and remove its immune tag in which case CDPR will have to make it more expensive and that's where the problem lies.
The strenght of Shakles right now and the reason why it's a fantastic card in HC (it's easily the strongest version of the card ever) is because it's a good removal (3 damage deals with quiet a few targets already and the lock ability kind of promote it even further) that's fairly cheap. If you make it more expensive I'm afraid the card simply starts being out of reach (especially considering immune is a very rare effect, it's kinda increasing the provision cost for very questionable reason, at least imo).
I don't have a big problem with the immune tag appart from the fact CDPR clearly under costed it. And that's the point where I fully agree with you, Eredin being able to give immune, boost by 4 and still keep 3 mulligans is all kind of BS...Don't you want him to sweep the flour and prepare the breakfast for the kids as well?
In fact, I think CDPR could hardly be more wrong about the mulligan they give to their leaders (no offense to them, that's a first shot, those things happen). Eredin and Foltest has the highest numbre of mulligans while they're easily the best leaders in their respective faction, Eithne has 3 mulligan with the best ability in the game, Arachas queen has only 1 mulligan when the entire game is built to counter her ability ect