Clearly it is at a good height now for imaging the whole surface, but as there is no atmosphere could it get down to a mountain scraping orbit? Just high enough to get round the lumps and bumps and variability in the roundness of the object? Would that enable it to image things at a really small pixel size?
Not really. The lower your orbit, the faster your spacecraft has to fly to maintain that orbit.
True, but in zero gee and with no atmosphere that isn't really a problem. It would take some time and reaction mass to get into such a low orbit. The problem is the uneven gravity field -- see my other post,