why would he accept a dude that's basically an outsider and clearly dislikes the cds? the only time they ever accepted an outsider is with harald, who was special due to how overwhelmingly strong he is and the fact that he's the ruler of elbaf and was willing to let them use the might of elbaf for their goals
If it's Imu, necessity.
What do you want to know? The HK are pretty much all just the descendants of the 20 kings and loyalists to Imu who start out as Gods blades who do missions until they become HK. The only exceptions are Harald and Loki. The main thing a HK has to have is slavish loyalty, they give up their freedom. Doffy is pretty much the opposite of that. Not trolling when I say that besides Rocks, Doffy is the only pirate known to have targeted CD's
We know very little of Knights individually. Shanks, Shamrock and Gunko were born all outside Mary Geoise. That may be coincidence, or part of a larger pattern. We simply don't know shit.
For all we know, there are no House Donquixote CDs available to use their ancestor's weapon - assuming besides Imu only a Donquixote can use it/use it best.
Doflamingo is very likely returning to the story and it's very unlikely to me he'll be a pirate again when he does, especially because there is a House Donquixote weapon up for grabs.