There is zero chance that Trump comes out of the meeting "changed." He's already agreed to help the GOP win the election, and to help GOP members with their races. He's always allowed others in the GOP to differ with him on policy and on legislation - he embraces the "big tent."
What he wants out of the GOP is their resources for managing a general election. he wants to be in charge of running that machine, but he's not necessarily interested in owning it. Think of it like a car or a PA system - he's the performer, he wants to use the house PA system, but when he's done with the performance, he doesn't plan to haul the PA system away. On the car example, he's just renting it.
I think most of the meeting will be meet and greet between the players who own the election machine, and the players who intend to run it. Policy negotiations come later.
I think congressional members would like it to be a greet and meet. I hope Trump goes in a
With a list of facts and future projections to put them all in their do-nothing places.