One could compare the shooting scripts to the episodes and test this idea that Shatner stole lines. Remember, there were claims that the Uhura, "Sorry, neither," line was ad libbed but it's right there in the script. It's certainly possible and likely Shatner's feedback on scripts resulted in changes, but on the set as-claimed is another matter.
I'd expect to see Kirk's lines in the final drafts, which would be tighter and more to the point than earlier pages with supporting lines all over the place. And Shatner surely had input.
But I also figure TV producers at large, and Roddenberry in particular, would give themselves the path of least resistance when somebody complained about losing dialogue. Don't be the bad guy, take responsibility, and put supporting actors in their place. Commiserate with the Takeis and Nichols of this world, share their feelings.
Put all the blame on the star, even if his script notes accounted for fewer line cuts than your own work on the re-write. Because you quietly agree with the star that the stories should focus on him, and you really don't care if people blame him more than he deserves.
And incidentally, if you want Nichols to keep doing the
naked-under-your-desk routine, she had just better blame somebody else for all her complaints. And you can't carry
that off if Takei and Koenig know the truth, so you have to lie to them about Bill, too.
One lie leads to another, like the time GR told Majel that NBC refused to have a woman as second in command, but he had fought valiantly for women's representation. And then he had to tell convention goers that story about NBC because Majel was letting it out, and she mustn't find out it was bullcrap. And then whaddaya know, GR is a feminist hero. Life is funny like that.
