re: why is it called a pro set offense?

This is indeed the classic pro set. It got its name in the late 50's, when most teams adopted it. The New York Giants were the last team to mostly use the old tight T-formation.

The second WR is the "flanker back." He is a back, so must lineup a step off the LOS.

Throughout the sixties and mid-seventies, the "fullback" ran the ball as much or sometimes more than the halfback or "tailback." Teams either lined the backs up as illustrated, or used an I formation.

Notable fullback-halfback pairings where the fullback carried as much as the half included Don Perkins and Dan Reeves of the Cowboys, Dick Bass and Willie Ellison of the Rams, and of course, Jim Taylor and Paul Hornung of the Packers.

Teams stayed in the pro-set most of the time because during the 60s and 70s, the active roster for an NFL team was 40 players, not 53 like today. Formation evolution was limited because you could not carry 7 WR's on the roster to come out with these 5 wideout sets, etc. The same thing was true for defense, but that's another story.