I never said Barthandelus was not on control.He was always on control.
Itīs just he could let a human rule over the Sanctum for a while and then change his mind.
A human primarch would be convincing and Barth could learn about human government by letting it.
After learning he could more easily assume the role knowing his personality.
But your alternative is also good.

I donīt get it Angrywolf.
There is a lot of information there.