The issue with the wolves is that there are two things happening at once; the Wolves are committed elsewhere - so you also need to know that you can afford to let Magnus sit around (which, in fairness, you could - he's not an active traitor at this point) to redirect the wolves.
On a similar note, the Emperor and the Ten Thousand turning up isn't possible for a similar reason - they're stuck either in the webway or at least in the dungeons.
Ironically, if Magnus hadn't done what he did, the whole Heresy folds like a house of cards.
As to the battle you've described, it's a massacre in the loyalist's favour. Seven legions to four is what the loyalist's plan originally WAS, and that was felt sufficient overkill to destroy Horus and his forces regardless of what tactical tricks the warmaster might have up his sleeves.
IIRC that was confirmed as not happening - it was in-universe gossip (and the Ultramarines aren't on his list of legions turning up).
It was confirmed as not confirmed, if that makes sense.
That's not quite the same thing. We know from The Chamber At The End Of Memory that whatever happened to the two primarchs, Dorn and Gulliman ordered a lot of the legionaries of the former II and XI spared (albeit with memories removed). That's not speculation, because it comes from Malcador himself, who was there and did the removing of said memories.
What happened to them, we don't know. The Word Bearer's theory is just a theory but it's as good as anyone's.
