you're allowed to ride up to headley overlook. the highlands trail frequently uses multiuse trails to connect hiking only sections. the picture of a mtn biker in the official mahlon map/brochure is taken on headley overlook, heading for the singletrack, so i wouldn't worry too much about it. the no helmets bit is questionable, as there are some slightly rougher sections between there and weldon rd. on the rail bed, whatever.
Nope, it is part of the Highlands Trail. Arothar says you can go to the Overlook and back on the double track, but you are not supposed to continue on beyond that point.