Given that Kinberg was one of the producers of First Class, it is likely he had creative input on the film and have been working with Singer/Vaughn in developing FC2's story. Given the stormy production of TLS, the problems of that movie were likely already in motion before the pen hit paper.
Kinberg was one of the producers on
First Class, and rumors circulated that he polished the script during principal photography, however Vaughn and Goldman's rewrites had a very significant impact on the story and the screenplay. Even though Kinberg claims he had no story input with
First Class and turned down an offer to write the script.
The problems with
X-Men: The Last Stand, from what I could ascertain, mostly fell under the responsibility of the creative team than it had to due with anything else. Yes, the production schedule was rushed, but Bryan Singer experienced similar problems with the first two films. With the first
X-Men film, he encountered budget cuts, constant re-writes, casting problems, and had a total of six months cut from his production schedule. Even with the short running time and all of these problems combined,
X-Men still turned out decently well (and is still commonly regarded as a good movie overall).
With
X-Men: The Last Stand, I think a lot of the creative problems had to due with writers Simon Kinberg, Zak Pen and director Brett Ratner. For example, when Matthew Vaughn was still attached as the director, the film was supposedly going to run over two hours. When Ratner came on-board, he made several cuts to the story, including cutting out several set-pieces. Such as Magneto and the Brotherhood storming Alcatraz Island and freeing several mutant prisoners. In the film, that sequence had been re-positioned as the convoy sequence where Magneto frees Juggernaut, Multiple Man, etc. Ratner essentially took the sequence of moving the Golden Gate Bridge and storming Alcatraz and made that the climax of the film; in the original story, the ending was going to take place in Washington, D.C. but Ratner felt like he didn't have the expertise to film that ending, so he cut it (proof of this can be found on the film's DVD, which includes an early, unreleased teaser trailer showcasing the X-Jet arriving in Washington, D.C.; Ratner also talks about this in the commentary).
Several other elements were supposedly cut from the screenplay, and in an interview, Ratner explains that he gets bored very easily, and wanted the film to have a very brisk pace, which is why there are so many characters and storylines in the alloted running time. Comparatively speaking,
X-Men and
X-Men: The Last Stand have similar running times, but I find
X-Men to be the superior film. I don't think rushed production schedules or short running times account for any lack of quality regarding
The Last Stand; I just think the film had inferior storytellers, at least in comparison to Singer and his creative team.
I think that's evident even with the studio, who offered Singer both
Wolverine movies and enticed him to return for
X-Men: First Class. On the other hand, Brett Ratner hasn't received such offers.