Separation of powers. Congress has to go through the executive branch in all foreign negotiations and agreements. We have the President, State dept, and ambassadors authorized to do this. Congress can only act to FUND agreements actions or not, they cannot bypass the Executive and go make their own deals directly with foreign powers or influence actions or outcomes.
They can’t go around the executive and throw a wrench in the works. Not in their “official” capacity.
So I am pretty sure that in his “official” capacity Graham cannot negotiate or influence. But in the capacity as a citizen he can go visit but can’t make any binding deals or influence outcome because that would then violate the Logan act.
I've been on this earth for 63 years and have yet to see them enforce that act. I have no expectation they will start with Linda.