^^^
WP supported the GSR storyline, often defended it, and he certainly was involved in all the decision making regarding the relationship while he was on the show. So, he had to be ok with the marriage, as he was there for the proposal. That said, when he left, he left. And to be clear about his intentions, he most definitely had a say in the way Grissom left...with Sara.
But I don't think he ever intended to return to the show. From what I can tell, he made an appearance in The Two Mrs. Grissoms for Jorja. He was good with continuing the marriage off screen, but had moved on IRL. So, it was up to Jorja and the current ptb to continue it, and they failed...big time. Jorja chose to stay on the show, and they chose to end it. Worst. Decision. Ever.
As for the finale, I think Billy's not one for cheesy; he prefers subtle, everyday life experience. He said in that interview that if given more time, they may have done it differently. But I do think he wanted Sara and Grissom to end up together, married or not doesn't matter. He came back to reconnect, to finish the story he started, and he did help formulate the end boat scene. No doubt the clichéd sail-off-into-the-sunset was done for the fans, and AZ. But it was a much needed happy ending for this long suffering (15 tv years!) OTP. And it turned out beautifully, imo. And this fan will be eternally grateful.
ETA: I remember that interview about the marriage. I recall thinking how it was poorly edited; and the 'not what he wanted for Grissom' had more to do with not wanting to return to the show, and not about the marriage. Most interviews about his leaving indicated that he was done being Grissom; he gave him a good ending in his original exit and now wanted to move on. And his interviews around the finale indicated he came back for the fans (so why not please the fans). He most definitely had a say in how his return was done, and specifically, how GSR was handled. And I think, looking back at old interviews, this was the way he and Jorja wanted their story to end.