Baratie-Group of close people who worked together in a kitchen, and treated each other like familyThose are the most hardcore pirate crews in history.
A village.
A Kitchen.
And a group of people that help you take apart ships.
I explained why Law isn't. Luffy hated the Pirates that left laboon. He HATED THEM. Luffy isn't the straightest arrow in the box so when he sees someone and auto asked them to go on an adventure dont take it to heart. They arent crew. Law isnt crew. Luffy thinks he is. But he isnt.
Franky Family-Group of thugs who were close and treated each other like family
Luffy's crew-Group of pirates who treat each other like family
Like I said, Luffy was angry at those pirates because he was under the impression that Laboon was abandoned, that they parted on sour terms. When he met one of those very pirates, and they explained what happened, he realized Laboon wasn't abandoned, the pirates were just unable to complete their promise. Upon discovering that the pirates left Laboon on good terms, what did he do? Boom, accepted one of these "hated" pirates into his crew.
It was never stated that Luffy hates people who abandons their crew, it was stated Luffy hates those who abandon their friends or people who rely on them. Usopp asserted Caesar is the kind he hates the most because Caesar abandoned and used those who relied on him. Last I checked, Caesar and his guards weren't a pirate crew, and seeing as how Usopp knows Luffy better than you do, clearly, it confirms that it doesn't matter if it's a pirate crew, a rag-tag group of friends, or a group of criminals, he hates you if you abandon who rely on you. Period. This collapses the very basis of your argument.
The fact that Luffy accepted Brook, Franky and Sanji is more than enough proof that Barto can join, regardless of if you want it to be that way or not.
And I know Law isn't a Strawhat. Explaining why is irrelevant because, as you said, Luffy doesn't understand why. It doesn't matter that Law isn't truly a Strawhat, what matters is that Luffy sees him as a Strawhat, which confirms that Luffy doesn't mind having people from other crews, even captains, in his crew. After all, Luffy has final say on who is in his crew. Shit, Shanks and Teach, polar opposite pirates, both have asked people who part of other crews to join theirs, with Marco and Ace respectively.
So enough of this "Barto can't join" bullshit.