It's a bit retarded, to be honest and a very bold move on Lyman's part, Garland/Willas can have the Retinue in that Keep, and basically remove him very fast...whatever the Tyrells have in the city, I would imagine that Lyman wouldn't. Garland doesn't care, if he is told he is not regent, he will do whatever the fuck he likes, and he will literally say "Fuck Aerys" if even Aerys acknowledges it for a moment, they know they can do it without mercy because Garland's practically the Regent right now, and gives Aerys his power, to a roughly equal degree as Drogon does...which oh, is surrounded by Tyrell guards. It is scary stuff, and most definitely, sees a real sort of moment where I will have Garland's force reply to that. Money is power, men are power, and while he is in a bed, still not very able, he is surrounded with Reachmen who are very, very much so able. It might get messy.
Tad edit. Oh, and that whole assassinating Aerys thing? Well, it got bumped up an order if said denial from Aerys happens. It's going to be extremely bloody for everyone, but I doubt he'd get to Drogon in time. The power is there, and it's going to be interesting...