going to have his own show or did they know the spinoff wasn't going to happen?

What was the purpose of Fergus? Whose brainchild was that guy?

The head writer George Tibbles was responsible for that 4 part Fergus mess. All his other season-opening story arcs were good; this one, pretty bad. Several episodes in this season were pretty bad (although this season has one of the funniest shows of the series, The Enthusiast), and the last season and a half were never syndicated to local TV; thus, never remastered to videotape from the network films.

As far as Grady goes, he was written out of the last season (per Wikipedia, he informed the show he was leaving), probably when the spinoff pilot, which was also pretty lame, wasn't picked up.

What was the purpose of Fergus? Whose brainchild was that guy?

Fred McMurray had been the Scottish Laird Douglas in Season 4, when it was really common for sitcom regulars to double up on roles, sometimes using accents. The show was on for so long that doing it in Season 12 was long out of style.

Here's a post I wrote nearly 3 years ago in this forum, regarding the marriages of Fergus and Steve:

I did like Mariette Hartley as Mary Kathleen in the Scotland episodes. Playing a character who's a prospective bride for Steve, I thought I would compare her age with Beverly Garland's:
Mariette b. 1940 -- Beverly b. 1926
So, Steve would've been paired with a significantly younger lady. Anne Francis was born in 1930, so while still very young for Fergus, not as extreme as a Steve/Mary pairing.

I also noted the lovely updo for Tina Cole's hair during Fergus' wedding, when we discussed what to like about those episodes in particular and S12 in general. Hot to the extreme...!

Fred (and Steve) would have been thirty-two in 1940 - Steve may as well have married Katie (Tina is roughly the same age as Mariette).

"What about Daddy-Daughter Day?" indeed!