MC would technically be a controllable husband. Also MC would "take over", he would just let her rule while he adventures.
Not unless the King decides that her usurping will lead to discourse in the realm. Especially if she is enforcing her rule with the aide of bandits. They are not gonna like a power shake up. I'm not saying a full army, but a small garrison to investigate wtf is going on.
Well if MC is introduced as a minor noble when he meets the Baron & plays his part then perhaps the marrage is arranged just before the Baron is killed. That was me completly spit balling a half-baked idea tbh.
Seems overkill.
Again, why raise an army to create havoc around the Peasentry's that normaly don't give a damn about the highlord they are paying their money/taxes for, if she is able to become baroness on a much easier way?
The King wouldn't bother whom a backwater baroness marries, as long as that individual has no ties to a neighbor or potential hostile faction/threat.
A Baron would be unlikely to just believe that an uneducated person that lacks any form of refinement to be a noble... no matter how minor, or atleast would disregard him as unfitting to succeed his title.
Considering that we already play several factions against each other and worm ourselves up their ranks another power struggle between 2 Half Sisters of Noble and not so noble Blood wouldn't be out of character.