They help a little bit to extend dry time. In my experience, Pro Classic is not a very user friendly paint if you are trying to avoid brushmarks. I like BM Advance or Aura better for their self levelling properties. I would only sand in between coats if the surfaces are rough to the touch.

I use floetrol all the time. It extends the drying time, so paint doesnt dry so fast, so yes, it does help minimize the brush marks. I would sand, all of the old paint, lightly at least, before painting just to make sure you get the best adhesion for a longer lasting paint job.

I was going to comment but cat_ky said exactly what I was going to say. I like to sand very lightly with fine sandpaper between coats.