Let's say for some reason none of them do. Einar Thambarskelfir was a powerful jarl in Norway, and he's the one who gave Magnus his throne! So maybe since Harald died down south and didn't kill him, Einar could be sort of a underking in Norway to help with that problem?
Why not? Of course, doing that would probably means that Magnus suzerainty over Norway would be more nominal, and it would be more of a Dane kingdom with suzerainty over Norway.
But it could work.
My plan was for it to kind of all been centralized by the time of his death (not fully), and for him to have a son to take over once he's passed. Norway would eventually get independence
That's definitely unlikely. Centralized and medieval states are mutually contradictory.
For a centralized state, you need a strong bureaucracy, an unchallenged rule, and no concession made to noble's power.
Danemark does not have the material, and I dare say, the mental structures needed for that at this time, as almost all the vassalic/semi-feudal/feudal states.
The best you could reach, and I point *best*, would be an unified rule without sub-kingdoms (de jure or de facto) in Danemark, while Dane nobility still playing an important and autonomous role.
His claim was that Hathacnut (Cnut's son), who was king of England at the time, made the deal that if one of them die, the other gets his throne. Harald based his claim off of that. (You probably already know this though).
But it's a totally legit claim, following the "rules" of germanic kingship. The sucession of titles wasn't strictly dynastic (even if the same dynasties often took the crown) but by the kingship.
Resuming it : if a king of Norway became king of England, his sucessor , even if hewasn't his son or else, could claim England as it's going along with the kingship of Norway.
Yeah, that could work. The Danish and the Normans ally and take England together.
Until tensions arose, of course. The division of England between two conquerors never ended that well.
Well, maybe he could peacefully negotiate with Sweyn (or kill him off) to get that out of the way, and get Danemark under solid control. With Einar and some of the Norwegian nobles under his thumb in that Sejm sort of thing, Norway would quiet down.
Well, he admittedly possibly settled with Sveyn, making him more or less a de facto co-king in Danemark. I would think that is not "solid control".
He could kill Sveyn, but Danes didn't liked the idea of Magnus being too much, and would propose lead to another candidate.
That doesn't mean Magnus can't prevail, but solid control seems out of reach.
And Einar being sub-king of all Norway, aka the power base of Magnus, isn't something I would call "being under his thumb" or "quiet".
And then if he is in England, with William's help they could reestablish peace there.
William isn't to be too much trusted : OTL demonstrated that he wanted as much England he could have and that he had the forces for that. I could see Normans pretexting revolts in Dane England to expand their authority.