Hmmmm... Totally forgot about Garp and Ace being Dragon's kid.You know, that I think about it, you can work it pretty well actually. There are a few things. Dragon is the son of Garp, that much is fact. Also, Shanks joined Roger's crew and Roger was not known to have any children besides Ace. So Shanks and Dragon are definitely not brothers nor are they the children of Roger.
However, they can be "brothers" in the sake of brotherly bonds like Luffy, Ace, and Sabo. This can work. Also, if that is the case, the reason Shanks is so fond of Luffy the way you explained is plausible. So if you take out what I said before, the rest of your theory works.
Basically, the plot twist would be the Shanks and Dragon are brothers because they shared some sake which parallels how Luffy, Ace, and Sabo became brothers
No. Just no.
Them not being biological brothers would take down the significance/plausibility of Shanks being a D, but it seems like everyone is D, so why not? lol And Shanks is uber good with Haki and has Conqueror's Haki, so possibly. And we don't know his full name, correct?