SuperCow wrote: Can this ability be used to reduce the coversave of a vehicle that is obscured (4+ save)?
I see no reason for it not to work as normal against vehicles. Nothing in the relevant rules specifies Infantry, or non-vehicle units.
Also, can the markerlight ability stack so that a cover save of a unit/vehicle will be reduced by more than one
Yes. Specifically covered in the Markerlight rules.