I won't be questioning the idea (because you know why haha) so I'm just going to comment on the details you gave. The Monkey family (aside from being a D) is very, very mysterious to me. Oda has not even once told anything about Garp, his wife, his kids, lineage etc. And I can only think of one reason, it is something important. Which is why I agree that Luffy's mom might have a really important role. Or maybe the entire Monkey family, who knows. I remember back then when someone asked Oda to draw Luffy's parents, he answered they'd be interested in the next volume or something, but it was only the introduction of Dragon.
Yup, all the breadcrumbs will eventually connect. So there's going to be a reason why it hasn't come up, and the pattern of genetics is undeniable at this point. It's kinda scary how what could happen is now exceeding what I was expecting to happen 2-5 years ago (however long it was when Dragon being Luffy's father was revealed and also the piecing together of the breadcrumbs). Even if she isn't a fighter, she is still outwitting a brilliant tactician. Either way, the entire Monkey family is destined for great things, and this fits perfectly with Dragon lucking his way into the perfect wife. It's his son that is going to become the complete "Joy Boy" and bring about Dragon's dream. Of course Dragon is going to be a huge help, so he's not completely dependent on riding Luffy's coattails, but damn...the ending of the series is looking AMAZING!>
But what is the possibility of him originally being a D but only took the Monkey family name?
If this is the case, then there is no need to seek out a specific wife. If he's already a "natural enemy", he has full respect from potential recruits and has the "fear" that gets put in the minds of the Tenryuubito when the war actually starts. This double win is the only reason why it is the most important step/foundational achievement in building his dream of overthrowing the Government. >
For instance, he could be like Law, maybe? I mean, yeah, the whole world hates Tenryuubito. But it's not like someone would just decide overnight he wants to go out and wage a war against the government. There must be some sort of a motivation. Which leads to the emergence of the theory that it might have something to do with Luffy's mom. But, i think it's also possible that it's because he actually has found out what happened to the ancient kingdom and void century, that motivates him to build an entire army.
Ebony Rustle too. Maybe I might have to phrase it differently. Yes, multiple D.'s are going to have the same dream. Even Luffy has the same dream, but Luffy's dream is results driven. The people get what's right from him overthrowing the Government (purposefully avoiding being too specific with Oda being extra sensitive to people discussing the "real ending"). However, the key plot point is how easy it is for Dragon to achieve his dream if he is a D.
If Dragon is not a D. member, Kuma, Iva, the other 4 RA execs, etc. might not join him because of the whole "D.'s are the natural enemy" to Tenryuubito. If Dragon already is a D., he has no need for seeking out a specific wife. He probably doesn't even have a kid based on how introverted he is. Ah, but the whole "destiny" aspect is WHY Dragon institutionally faces towards Luffy. Luffy is the only way his dream gets achieved. At this point in the series, Dragon is smart enough to see the pattern of his son's achievements and the only path he is going down (meeting Robin HAS to be the nail in the coffin in this regards, which is also why she got one of the biggest power-ups => she got their best training once he figured out who she was and how willing she was to help Luffy). Making a complete clone of herself seems impossible based on the initial mechanics of how she explains her power.
To give other analogies: if Luffy never meets all nine other crew nakama, his dream will never get achieved. If Blackbeard never gets the Yami Yami no Mi, he never progresses to a further step in his plans (they all know the plan and state they go their separate ways if he fails to obtain the Gura Gura abilities). Dragon becoming a D. is the most important phase of his dream. Blackbeard and Dragon are consciously aware of it since they are intelligent/plan everything out. Luffy's just lucky and has destiny on his side. Hopefully, some combination of this gets the key point across.
What are you basing this on btw? Not sure about this since we know nothing about Vegapunk, but I'm betting he knows more than the government does.
Vegapunk is the Governments absolutely most prized "possession". He is going to have the best security around him (and might be close to the new Marine HQ or in it for all we know). Garp still has MAJOR pull/control in the Marines. It isn't all that hard for him to request that they build an extra room in the same facility. Because Luffy's mom has not escaped, she is in something beyond what she can overcome alone. Vegapunk is in the same situation. Admirals being close by is a necessity of this "maximum security". This is the only connection...but it fits perfectly. If it was Impel Down, she'd eventually ally with the Okama. She should be able to take on and win against Shiryuu or Magellan if she could fight with Garp. The security has to be so good that she cannot get out all these years.>
I agree that Garp's indifference while revealing the identity of Luffy's father leans heavily on him possibly having no biological relationship with Dragon, but I'm doubtful with the naming. I mean, it would make sense if Dragon only added the "D", but why also the "Monkey"? If it was to exploit the prestige it got from Garp's heroics (which is likely the only source for it given Garp's personal background), it seems too opportunistic of a move for a person with such deep-seated views on social issues. And if that was the case, then I would have expected a more emotional revelation from Garp, perhaps even to the point of denouncing him and his use of the "Monkey" family name.
This is purely Japanese culture. "Makuyoshi" are men that marry into a "higher" ranking family that does not have any male sons. They are adopted sons that take on the wife's family name. Because Monkey carries all of the desired traits, this is why Dragon takes on both the "Monkey" and the "D." names. He only cares about the D. member side, but based on the adopted marriage, it has to be both. His real last name isn't a real D., so people that do enough research would figure it out and know he's lying. Ah, probably gotta add this to the main theory, oh and GoldenVenus since this might help clarify your first question. I'm gonna tag you in the earlier part of this response because Garp's reputation has nothing to do with this.