So Mando has the Dark Saber (DS), but cant control it because he is not the rightful owner.

To fast forward in ownership of the DS, let's start with Maul.

Maul was defeated by Obi Wan so the DS is rightfully his. Obi Wan lost to Vader, who then was killed by Palpatine, making him the master of the DS. Palpatine was killed by the Death Star. Yes Vader threw him off but it was the sudden impact that got him, so technically the DS belonged to the space station. Lando blew up the Death Star, making him the rightful owner.

So if this is all to be true, Mando must fight Lando to get full ownership and control of the DS.

Mando v. Lando... has a nice ring to it.