Lens flapping adjustment possible?  

Placement of thin washers at the lens mounting bolts you can effect flapping. Just be careful not to drop those washers. However, you still can't correct peripheral vs center focus like on a FPTV

Cost. The RPTV lenses only have to focus in a small range of distance values they can get away with using a simpler lens arrangement optimized for a single distance and still yield acceptable edge focus. FPTV need to contend with a large range of projection distances so the lense needs to cope with the varying curvature of the focus plane as the projection distance varies. That's why both edge and center optical focus elements are needed in FPTV.