I think what it comes down to is that Kirby really hated the Marvel Method and had a lot of bitterness about past events which made him reluctant to work that way again, even with a more equitable financial scenario.  Also, we don't know how much money he was offered to co-plot... maybe he didn't think it was enough, or perhaps he feared that "co-plot" would involve more than 50% of the writing work or even turn into "sole plot" as it had with Stan at times.  It appears he wanted a really clear division of labor and firm boundaries that would prevent him from ever being asked to do more than he was getting paid to do.

As to Roy turning it down... yeah, that is amazing to me, too.  As much as Kirby hated the Marvel Method, it appears Roy really did not want to work any other way at that point.  Admittedly, it would have created more work for him to have to write plots all by himself and fully break down the story too, but to me it seems like it would be worth the extra work to have a chance to do FF with Kirby.  I guess Roy didn't see it that way, though.

If I may be so bold...why? 

DESTROYER DUCK #1 penciled by Jack Kirby and inked by Neal Adams

DESTROYER DUCK #2 penciled by Jack Kirby and inked by Alfredo Alcala

DESTROYER DUCK #3 penciled by Jack Kirby and inked by Alfredo Alcala

DESTROYER DUCK #4 penciled by Jack Kirby and inked by Alfredo Alcala (with Val Mayerik)

DESTROYER DUCK #5 penciled by Jack Kirby and inked by Alfredo Alcala

Jerry Siegel wrote the back-up about an alien child--THE STARLING, with art by Val Mayerik.

Cover by Jack Kirby. Can't find who (if anyone) inked the cover.

I have to wonder if It was actually re-penciled or if Royer simply inked a new version on a light table.

 

Side note: man, the coloring on the MGC 77 isn't a patch on the original FF cover.  That red on the Mad Thinker is so beautiful and strikes me as truer to Kirby's intended vibe, and it's overall just a better-balanced palette.