Should lintels above wall portals be considered for smoke alarm placement?
Yes and fantastic question, the lintel/header/apron will absolutely delay detection...by quite a lot. Therefore, I absolutely agree with Jack that you need to place the detector closer (the lintel side that's closer to the threat) to the known threat area or even just within the shortest path from that area. Which, may not involve this lintel nor these rooms.

An abundance of appliances is always your biggest culprit indicator, not that a TV or outlet's wiring could be faulty... though they would've typically revealed themselves with early symptoms.

I would suggest either a ceiling placement of the detector or a high wall placement that results in the detector being very close to the crown molding. The ceiling will fill with smoke long before it escapes beneath the lintel/header/apron.