Well Jay n Silentbob, it's funny that you'd mention that.
While watching Baccano!, one of the things that I constantly had to keep in mind so that I wouldn't nitpick and overspeculate was that it is based on a lengthy light novel series (Basically illustrated novels. Not quite manga, but not quite novels either.), meaning that inevitably a hell of a lot of things are going to be left out.
There are extremely thorough follow-ups on everyone who appears (and survives) and everything that is pictured in the anime as far as I've seen, but they simply didn't have the time to jam it into that particular adaptation.
Unfortunately, the likelihood that we'll ever see the series (as an anime, anyway) continued is very very low. Evidently the anime bombed horribly in Japan but the light novels continue to be published. What's even more unfortunate is that it isn't likely that Baccano! will officially make its way to the US, and if it does it won't be any time soon. This is speculation though - there is a slight bit of promise since another of the same author's works is currently being translated.
Everything you mentioned, though? I've flipped through the light novels (I can't read a lick of Japanese, so I have to rely on pictures) and seen what appeared to be significant follow-ups on everything and everyone you mentioned in your complaint.
A particular favorite which I didn't actually see you mention is:
! The blonde woman with the spear who strikes Isaac's ear at the beginning/end. She makes a significant appearance in at least one of the novels.
Also, covering some of your issues more specifically, based on the illustrations I've seen while leafing through things:
! Ladd is confirmed to be alive - I believe he's in prison at one point. I don't think he ever gets a chance to confront Vino/Claire/Rail Tracer again.
! Some of the novels themselves take place in 2001 - all of them are dated, and go through a good chunk of the 1930s and skip to 2001.
! The man and the little girl definitely appear again, but I honestly don't remember what they were doing or if I could even tell.
EDIT: Firo's significance or lack thereof:
! Actually, Firo is a very prominent character. I believe there's a drama CD centered around him, even. His relationship with Maiza is incredibly important to the plot, because keeping in mind what Isaac said at the beginning about eating beef and stealing sacks of money: Szilard ate Maiza's brother, and Firo ate Szilard. This means that Firo has half of the knowledge of how to make the immortality elixir as well as the rest of Maiza's brother's knowledge and experiences. He also had the ability to restore Ennis.