IMHO main drains are worthless and are more of a risk than benefit. They don't add much at all to circulation and just add an entrapment hazard. They do not substitute for vacuuming and have very little influence to dirt around them. In fact, if you pour a bunch of dirt directly on a main drain, it will only clean about 1-2 inches around the drain and that is it. They also easily clog with leaves. People give MDs way way too much credit. I would skip it if I were you.

Dynamo is a great pump for an AG pool and a 1.5 HP is likely over sized if anything but it depends on the filter size. You don't need an IG pump. IG pools need IG pumps because IG pumps are self priming and are above water level. Although you can use a Dynamo on a IG pool because it just so happens to be also self priming.