Thanks for the replies everyone!
Actually, the mafia boss theory is even more plausible than the cop theory. I like it.
As far as the cooperation of the Rogues and the cops is concerned, there are a couple of things to consider:
(a) Not
all police officers would have to be in on the assassination, only a few at the very top, and maybe a few that were on the scene. In fact, other officers in outlying precincts might not have a clue at all, and might only know what we as viewers know. The suspicious thing to me is that they waited until
after the assassination to bust up the conclave, if they knew Cyrus was going to deliver a message of unity to rally the gangs against the cops, why not bust it up
before his speech to prevent him from getting the word out?
(b) The cops who did work with the Rogues might not have been thrilled with their alliance, but might instead have seen it as a necessary evil, "sleeping with the enemy" as the case may be. If they went strictly by the books and arrested Cyrus at the conclave, they may have been able to trump up a few charges that would put him away for a couple of months. But to have him killed would rectify the situation permanently. And with Cyrus out of the picture, who would have the charisma to organize the troops in his place? Certainly not Masai.
(c) It's entirely possible that Cropsey and the rest of the Rogues had no idea who they were working for, since Luther is apparently the only liaison and he doesn't seem too concerned about delivering details to his fellow gang members. It seems he tells them just enough to keep them on their toes and dangerous, but that's it.
I also contend that Luther had no intention of pinning the blame on anyone else until he noticed that Fox noticed him. From that point he knew he'd have to pin it on the Warriors somehow, so at the very least it would be a "our word vs. their word" situation to the other gang members. As it turned out, seeing Cleon standing near the Riffs provided him the perfect opportunity.
BTW, I've also finished the game and the novel, so hopefully I'll have more opinions to share once I bust out of New Blood territory.

Laying down strange wool since '79.