My 2 cents...do not compromise your head locations due to trees other than to make sure your pipe layout does not impose on the established root system. Last thing you want to do is kill your tree. Good rule of thumb, don't trench within the width of the tree canopy. Otherwise, design for double overlap spray coverage and work around it.
Place heads around curved walls just as you would straight, following the pattern of the wall at a double overlap distance based on your through radius. Make sure to turn heads all the way clockwise and install with spray pointing right and parallel to the wall and adjust arc from there based on your curve after installation. Won't be perfect but will work fine.
Use rotors on 27'x50'. You will have a throw of 27 feet..too much for sprayers.