The thing is that the power of the Yomi-Yomi no Mi doesn't happen until the first time its user dies. Thus, when Brooke died, his soul remained on this plain and he was able to come back to his body (even though its all bone now) Maybe if we knew more of the dynamic of the fruit and how Brooke could run across water (since he didn't sink in his bony state even though he supposedly can't swim), then maybe I could understand a bit more and perhaps you could have a case. But for the moment…
I just wish there was a bit more on the DF dynamic that Oda could explain. I would love it if he would flat out say if there was multiples of the same fruit and how certain ones go out across the world, but that would involve knowing where the DF come from in the first place.
Oda can't do that, it's probably something really important to the plot. If it wasn't he probably would explain it in the SBS.
Anyways, even if the DF doesn't activate untill death, it still is there. Brooke said he couldn't swin even in his first life. Another note-taking fact is that he looked for the body for a year. That's enough for another DF to grow untill he used his DF again to resurrect.
And as far as we know, all the DF can do is give the user another chance to live, no matter what happened to his body that killed him or after the death. No more to the DF. However, we still don't know what'll happen if, say, the head was cut off or something. In that case, will the user have his head dislocated from the body forever, or will one half work while the other won't?
If the head will stay alive dislocated, Brooke can probably dislocate his bones, since they're not really living or connected by blood and stuff.
And yeah, he can run on water because he's 1) "dead bones" light, and 2) because of his skill to move his legs like that [probably fighting style].
One thing to look after is his bounty poster, as it's been said in the chapter topic. "Dead or Alive" is gonna be somewhat hilarious. The picture should be funny too, a goofy happy skeleton jumping around.
Impel Down: Probably, yeah. I'd say almost definitely it's something that's connected to his bones and body [expecially his light self]. He WAS alone for a bit less than 10 years with the idea of beating the one who stole his shadow. He wasn't strong enough to counter him before his shadow was stolen, so his only choice was to upgrade his fighting style using his new body.