Shatner didn't write Kirk.
But writers do look at what the actor does and what his strengths might be and tailor the scripts for that. Which is why (IMO) Kirk became less like Pike as the series progressed.
Agree.
There's no doubt Shatner brought something unique and compelling to the role. He was highly memorable and appealiong as Kirk in a way that Hunter, possibly a better actor, was not. Certainly the revamped series, scripts and concept all contributed, but I would never underestimate the wisdom of casting Shatner as Kirk, the right actor for the part.
Shatner was very proactive (even aggressive) in shaping Kirk with producers and writers and highly protective of the character, especially as the series went on (not to mention jealous of Kirk's screen time vs Spock et al). It's easy to get annoyed with the Shat's mannerisms and OTT performances in season 3, but the fact remains that his is an indelible protrayl of an iconic figure. Larger than life really worked in this case.
I've watched the series for over 40 years. Recently revisiting the Hunter as Pike material, I have to say I'm glad things turned out the way they did.