No confusion. The Master seeks power, and retains much through the use of ancient and alien technologies. My expectation is that he routinely scans for Plodex life, and thus would have found Marinna in her false ddeath under the sea.
Realistically, a new mate would take time, and is not something the Master solely dedicates his time to, as seen by his reaction to Kang in the Avengers arc.
As for Namor's attraction to Marinna, Namor is one of the few Marvel characters who has a type--specific characteristsics he likes in a mate. Marinna, Dorma and Sue Storm (and Sandy from the latest Namor series) are all confident, capable women who do not let their strength of character diminish being feminine and ladylike. All can defer to their man without losing their independence or sublimating their personality into his. Judging from his mother's ventures to the surface world that led to his birth, the sexual interests he's had all remind him of his mother. And as a royal personage, it makes sense that Namor would pursue women who can act ladylike and appropriate yet be strong enough to make decisions in his absence.
Any return of Marinna should deal havily with Namor, and I hope Lobdell or any future writer remembers that most of Alpha Flight might agree to keep Marinna's return a secret from Namor, as a multiple-time widow, Heather would REFUSE to keep such a secret.