Jack Brabham didn’t do too badly either did he? And yes Bruce Mclaren in both F1 and Can-Am racing. Although both had mechanical engineering backgrounds, Brabham in particular and with no one really knowing much about aerodynamics in those days it would have been easier I guess. Wasn’t Enzo Ferrari himself a racing driver, although that might have been driving other peoples cars I’m not sure.
As for Fittipaldi I think we can assume he would have won 1976, he was better than Hunt and with Lauda’s crash he would have been challenging beforehand and there to pick up the pieces. That late 70′s, early 80′s era was very open, particularly once Lauda retired for the first time and Peterson was killed.
You had a different champion every year from 1977 to 1982, with the exception of Lauda all first time champions (although Mario Andretti was a racing veteran at this stage) so a driver with Fittpaldi’s experience would have been invaluable I’m sure. Mclaren didn’t actually win the title again until 1984 though and I think that’s pushing it to say Fittipaldi would have still been around driving for them. So I reckon 3 titles minumum. Anymore than that is debatable. He fell out with Chapman when he left Lotus so he probably wouldn’t have been driving for them in 78, so he would have to have bagged a Williams or Ferrari drive in all likelihood. Perhaps a Brabham or Renault although Prost couldn’t win a title in a Renault either. 1981 and 1982 were so open though that Fittipaldi in a decent car probably would have had a chance although he would have been at the end of his career by then. But then he won the Indy 500 in 1993 so he clearly still had it.