Is there any reason to expect Magnus the Red to be on Terra? I checked Index Astartes but it doesn't mention it at all (and also has Daemon Magnus fighting alongside Horus at isstvan V, so . . .)
He was on Terra in William King's old piece, where he lead the Thousand Sons in the first wave trying to take the spaceports, alongside Angron, Fulgrim, Mortarion and their legions.
Yes, the fluff has always had the 4 god-aligned legions spearheading the Siege with the SoH as the reserve force that Horus planned to commit once the walls were breached. I am sure the new novels will give a bigger role to the IWs in making that breach although it is not clear how much the NLs or Alphas will participate.
Lorgar also assured Horus in one of the novels that he had forseen Magnus fighting at their side on Terra so his presence at the siege seems highly likely.
If I recall correctly, I think Magnus only joined Horus in on the Siege because he wishes to reclaim a shard of himself that is on Terra.
That is definitely the implication at the end of Crimson King. Poor old Magnus, we already know there is nothing left for him to reclaim. It would not have made any difference to the siege but I wonder how the subsequent history of the 1K Sons would have gone if Magnus had been fully restored rather than remaining a fractured entity? Perhaps it would have made no difference. They were too far gone along the road to damnation to turn back, even if Magnus had recovered that shard that contained his nobler self.
Sanguinius stood up, stretching his wings to their full extent. He flexed his hands. "I need no blade".
It was as though Sanguinius gleamed with pale light, his face white, eyes becoming blood-red, surrounded by the golden crown of flowing hair. Guilliman had witnessed glimpses of of his brother's wrath before, but had never seen the true Blood Angel unleashed. Sanguinius surged forward on alabaster wings, half a meter from the floor, whiteness streaming from him like flames.