Magnus died in 1069, so any fight would be brief.
You're not really accounting for butterflies here, it's not as if he died of natural causes.
Maybe Olaf marries Edgar's sister.
Possible, there's the precedent of Emma of Normandy, Æthelred's widow, who later married Knut.
Margaret probably still marries Malcolm III of Scotland, but there's another sister, Christina, who was unmarried.
Olaf might, however, still prefer to marry Ingerid of Denmark to any member of the effectively displaced house of Wessex, especially considering that would prevent an invasion by her father Sweyn II, who was planning on pressing his own claim on England.
How did he die? It may be changed by the POD.
A fungal infection derived from eating bad bread. Butterflied away pretty easily.