from everything in this thread, my guess would be the Chiefs giving him a try before the Rams. The Chiefs and Andy Reid are known for giving troubled players another chance...which has worked and not worked but it is more the culture than here IMO.
Second, they have more of a need. #10 seems to be getting injured more often and the Chiefs are more reliant on their WR corps than the Rams. Mahommes fits perfectly with what AB provides on the field. As a weapon in general, AB is a great talent to be sure but featuring any particular player doesn't seem like part of McVay's system.
AB is better than Cooks but when Cooks has been healthy, having that speed hasn't been a night and day difference for the Rams. Adding a better Cooks doesn't seem like a missing piece to me but I can see argument and temptation to acquire a talent like AB. But his talent doesn't exist in a vacuum and that's ultimately the problem.
Terrell Owens was probably the greatest "talent" the league has ever seen and even he wore out his welcome. Team chemistry is a tricking thing and bringing in AB would send a ton of messages and signals to the rest of the team that probably wouldn't be ultimately beneficial. At best, AB would be a 6(10) game rental who would only be here to rehab his own image. He would not be brought in as a part of the culture or the future. Which would strain the credibility and integrity of McVay and his message.
Don't you think it worth noting that of the examples you gave of risky personalities that McVay "controlled", all of them are gone?
And I echo the poster who said if the Patriots and their team and culture couldn't keep AB in line, I don't have hope of ANY team being able to. Even McVay.