Your observations overall are great, but I would add a few of my own.

I thought the pneumonia illness was a foreshadowing of Fredo being "the weak one" more than anything else. I don't think the disease "caused" his ineptitude. I agree that Vito looking so forlorn was a sense of helplessness on his part, a feeling he did not like.

I agree with you about the way Sonnt treated him in the scene where Michael says he is going off to fight in WWII, and that Sonny probably had beaten Fredo up before. Note in that scene one of Sonny's kids says "Daddy's fighting AGAIN."

His being drunk at the wedding again shows he is the weak one, unable to stay sober at his own sister's wedding, and making a general fool of himself in front of Kay and Michael.

As for his attending the meeting with Sollozzo, I think Fredo had the sense to keep his mouth shut while his father was the Don. He knew Sonny was the heir apparent, and he was content to be more or less his father's factotem, and Vito probably was happy to have Fredo close by so he would stay out of trouble and remain safe. Of course all that changes when Vito is shot.

In the novel it says he is sent to Vegas in part because he had some kind of nervous breakdown after he failed to protect his father as well as the fact that sonny wanted him out of the way.

IMHO the whole scene with Michael and Moe Green shows us that by that time Fredo was really clueless about what was happening with the family business. He was fat and happy in Vegas, and had probably convinced himself he and Moe were really "good friends." He has no clue that Michael has already taken over. When Michael says he is buying Moe out, Fredo is incredulous...he says "are you sure about that? Moe loves the business....he never aid anything to me." Then when Michael
tells Moe to come up with a price, Fredo interferes and goes to Tom and mistakenly says to him that he is the consigliere, which of course he was not at that time. Even then after Moe leaves, Fredo admonishes Mike for coming to Vegas and talking to a man like Moe Green like that!

I think Fredo sees Mike as a kid tellinig off a Vegas bigshot who he (Fredo) himself fears, and he is totally taken aback that Michael is now running the family business. I don't think he was smart enough at that point to understand that there was nothing in it for him.

Whether or not Michael could have handled the "Fredo problem" better has been the subject of other threads and postings, so other than to say I think he could have, I'll leave it there.