What if some other loyalist Primarch returned to rule as the Emperor-regent instead of Roboute Guilliman?

[Tyzuris]

So if instead of Roboute Guilliman being brought back to rule as the Emperor-regent, one of his other living loyalist Primarch brothers is brought back in his stead.

Scenarios:

1) Vulkan

2) Jaghatai Khan

3) Corvus Corax

4) Lion El Johnson

5) Leman Russ

6) Rogal Dorn (evidence is not conclusive that he's dead, so assume he's alive).

How would each of these do as an Emperor-regent?

 

[CarlManvers2019]

How good are each in-terms of politics and administration?

And the question isn’t just about skill or intelligence, but the sort of personality to deal with it all

 

[Hlaalu Agent]

1) I'd say he probably does pretty well.
2) Since he is space Genghis, he does very good.
3) Pretty sure like most of them, he does at least decently.
4) Decently, if he isn't actually the real traitor.
5) Yeah, no. Not cut out for it.
6) I think he does good, but he has problem thinking outside the box it seems.

Best, I'd say are either Jaghatai or Dorn. But none of them are rowboat, they aren't fitted to the task by nature, and if he is a rowboat in this, the others are either rafts at best, or that door from Titanic.