Oh hell, so much for smiling and nodding...OK, here we go.
If you want a more accurate gauge as to what extent a keeper can exert muscle control, remember what it did with Captain Jack on Mars. It wouldn't let him directly warn Marcus and Franklin, but he was still able to drop numerous hints. Then when he's face to face with Tessa, even though he tries to shoot her, you can tell he's clearly fighting it tooth an nail. His arm is stiff, his hand is shaking and he's taking his sweet time pulling the trigger.
Londo was able to repeatedly knock the thing out over the years by getting hammered. It's a weakness the Drakh are even aware of since Londo mentions during his visit to Minbar that he's been denied alcohol. Indeed, why do you think he seems so desperate for a drink? He wanted to knock the thing out so he could warn them without it killing him. And before you ask, Londo knows that if he dies, they'll just find someone else to prop up on the throne and they might not be able to save Centauri Prime in the long run.
As for Drakh baring keepers, it kind of is what you think. They're created in these little cocoon like husks until they reach a certain stage, then they can be attached to a Drakh when it forms it's symbiotic bond. Indeed you see quite clearly in the scene where Londo gets his keeper where it came from. Again though, these things aren't very bright. Maybe dog level intelligence. Which is something of an apt comparison as what a keeper *really* is a collar an lead and the Drakh is the one holding the lead. Yes there's a degree of control, but it's nowhere near as total as you seem to think.
Mostly it controls because the Drakh is clever and cunning and knows how to manipulate their subject into submitting. With Londo it was a threat against Centauri Prime. With Captain Jack I imaging it was a threat against his daughter. The Regent....hard to say, but I think in that case it was brute force intimidation and terror that really broke him.
And finally; OK, sure, you can ignore the keeper in the urn all you like. I'm sure the reason David was on Centauri Prime with his parent hot on his trail had nothing at all to do with it.
