Next 4th Doctor story should be The Sunmakers, which was made before Fendahl but run after: it's a Robert Holmes script, the last he wrote in his time as script editor (which might be why it was run after Fendahl, as it made it easier to pretend he wasn't double-jobbing, which was banned outside of emergencies: which is why there were six Holmes-written scripts which were either officially emergencies, or credited to pseudonymous writers over the previous few years. Alternatively, they maybe wanted to run the 'ghost story' Fendahl at Halloween).
Fendahl was Boucher's last story because he became script editor on Blake's 7 (after Robert Holmes turned it down, as he he'd just done four years on Who, and suggested his protogee instead), and the head of department told him he couldn't work on both (a half-written script for the next season was abandoned as a result, though bits of it may have later turned up as one of Boucher's Who novels. As he told me 20 years ago, he can't remember what was in it, but was fairly sure that it probably leaked out into some later idea). Holmes and Boucher tended to swap jobs like this for the next 10 years, across Shoestring and Bergerac and others (if Holmes hadn't died in 86, I bet he'd have written for Star Cops).
Personally... I didn't like Fendahl at the time (age 10), but love it as an adult (so a total inversion of Invisible Enemy). That seems to be a common reaction from people who first saw it as a kid.
But whatever else we might disagree on, I'm with you 110%on Troughton's Doctor. So good you wonder why he doesn't scare anyone else from taking on the role. "I can never match that." (watching some of his slapstick bits. "Phew... Ok, so he's not always brilliant, just sometimes funny. I can do funny.")