Yeah, you probably won't be able to mow that low if it's bumpy, but that's where the top dressing comes in, because that will level out the ground and make it smooth and flat to mow on. The thing I've noted about my SB, and likely yours too, is that they can be set low like a bowling green, but also the highest setting is I think about 25 or 30mm, so you could probably mow it high with the Scotty as well. Also the roller will help level it out but it won't do as well as a good top dress. Probably best to top dress it first. Personally I'd go with the sandy loam rather than straight sand, because it has a small percentage of soil and clay which hold nutrients and also bind it together.
The reason I mow short in summer and long in winter is because couch is a warm season grass, so it grows at its best in summer, it can be cut short and still look great, it also rains more. In winter you're trying to keep it as healthy and green as you can because it's the grass' "off season" if you will.. if you mow too short it could well brown off.
