This part doesn't make sense
Magellan wasn't child of destiny, but if there was no Shiryu, he would have ended Blackbeard Pirates' career there.
Shanks too, if he wanted, and if he was worried of Blackbeard that much, he should have defeated Blackbeard Pirates right there at Marineford. Or any other times before Blackbeard becomes too strong.
If he just waits and do nothing until Blackbeard grows strong enough to beat Shanks, that sounds really dumb move from Shanks' PoV.