AFAIK, the Merlin isn't
throttlable, but I could be wrong about that.
More importantly, the Merlin runs on RP-1 and Liquid oxygen, a precarious mix for a spacecraft trying to land on the moon. That, plus the lighter gravity of the moon means you'd be better off using the landing platform with a cluster of five or six superdracos or a modified Kestrel engine fueled by hydrazine.
If SpaceX was smart, though, they would probably take a page out of the old Soviet playbook and have the landing platform and its propellant tanks as a separate unit from the actual engine. This way, the ship blasts off from the landing platform using the ascent/descent engine and leaves the spent propellant tanks and the landing legs behind. Saves weight on the descent that way.