I'm not sure where you're getting this 60% of all available Space Marine numbers from.
60% of M.41 Loyalist Chapters derive their geneseed from the Ultramarines. So...not very relevant to the current conversation.
I disagree with Curze. It might work on the short term, but as we saw with Nostromo, as soon as you turn your back people will revert back to whatever they were.
I STILL think Horus is the right choice for Warmaster. He just got along with most of his brothers. And the ones he didn't, they merely were indifferent. I don't think anyone overtly hated him (jealous of his elevation to Warmaster, but not outright hatred).
I would break up the role of Warmaster, though, and give the administrative portion to Guilliman. Maybe give the information gathering to the Alpha Legion (however much they'd be willing to share is another matter...)
I'm on the fence about the Thousand Sons researching the arcane. On one hand, if you keep an eye on them, they could discover a wealth of information. On the other hand, they're probably going to hide whatever they're doing. I don't see it really working unless Magnus buys into overseeing the works of his sons.
In short, I don't think forcing the Warmaster to wear so many hats at once will work. The job is too vast for a single person--or demigod. You'd be better off working with a group of individuals working in concert. Maybe call them the Elevated Nobles of Earth or something. That ought to work.
But if I HAD to pick, I'd still choose Horus, murder Erebus, and purge the Word Bearers.