Rossy flew Mirage, Tigers, and Hunters, 747...not sure what he currently flies for Swiss, but he is definitely a qualified, professional pilot, well aware of risks and more than familiar with meticulous planning.
His first flight tests of his carbon fiber wings and RC-model jet engines were probably - from a conservative viewpoint - a bigger risk than what he is doing now, after continuously improving his setup, and with more experience using it.
I agree that wake/wingtip vortices may not be such a huge danger for a human-sized flying object equipped with a parachute. This formation flight was probably safer, better planned, and better executed than many other flights worldwide. Amazing to watch
Regarding manual flight and Mk 1 eyeball: according to himself, he controls his "flight control surfaces" (i.e. his body) with his eyes. He looks in the direction of where he wants to fly, and his body moves accordingly, a very intuitive process. Flying by the seat of one's pants.