As to the Crowley-Mika alliance, the first prominent problem is why would Mika trust Crowley and vice-verse? To Crowley, Mika is just some kid unfortunate enough to catch the vampires’ attention enough for them to turn him into one, and to Mika, Crowley is the right hand man of his most hated foe, Ferid. Even now, they’re barely aware of each other, forget in the past. There’s just no grounds for them to team up on. It might (might, I emphasize) become possible for them to compare their situations and reach some kind of agreement now, after the current events in the manga, but during those past 4 years before now? No way. Just no way. There are other problems, too, like his death being not the only thing they want from Ferid, information takes priority there, and Ferid being stronger and smarter than both of them and all that entails. Even if a fight happens, I don’t think they will find it 'easy’ to win; Ferid has more speed than them, all he has to do is to quickly render the slower one, that is, Mika, immobile, and then it’s back to a one-on-one between him and Crowley and we saw how that ended, twice. Crowley has Chess and Horn who he could probably use if ganging up on Ferid really was all it took to take him down. But it’s obviously not enough, not with Ferid. In that sense, he’s a lot more troublesome as an opponent than the top progenitors, because he’s smarter, possesses fearsome insight and extensive knowledge of human/vampire nature and not nearly stupid enough to underestimate his opponents.