In this strange complex of time and space anything can happen

The thing I really liked about Spearhead from Space was the way that it explored the Doctor's more childish side, which often reverses the dynamic between the Doctor and the Brig, with the Brig cast as the stern but indulgent schoolmaster, and the Doctor the naughty schoolboy. And as you so rightly note, they return to this dynamic in Robot.

With Inferno, the thing that really got me, and made this memorable, was the fact that the parallel world is actually destroyed-- I think that makes this one of the darker Pertwee episodes. I also thought that it was an interesting exploration of fascism-- when we encounter fascist groups in Invasion of the Dinosaurs and Robot, they tend to be working from environmentalist principals in a misguided way; it was good, here, to see a fascist group who simply were more efficient at destroying the Earth.

I agree with others that The Green Death is well worth checking out-- it's one of Jo Grant's strongest episodes. But really, I don't think that Pertwee's era really finds its feet, and starts producing consistently good episodes, until it's final season with Sarah Jane. :)