I don't have any information about what they're doing to the scenery aside from the art they've released, so I can't comment either way on the riverboat's fate. All I can say is that it seems like a really pointless, wasteful and just outright baffling decision to get rid of it. There's literally no good reason, it fits a Tiana ride even better than a Song of the South ride (riverboats are actually prominently featured multiple times in PATF, whereas there aren't actually any in the original SOTS).
The claims about the animatronics from the finale also seem a bit dubious to me. Not based on anything I was told either, but just the sparse bit of media Disney has put out about the ride thus far. If the ride was likely going to be generally sparse of AA's, then I would be more inclined to believe the claim that the finale would receive such a drastic reduction. The thing is that there's some decently strong indication that the ride will still have plenty of AA's in the other scenes.
The most recent piece of art they released late last year for WDW's variant showed the first interior scene right after Slippin Falls. This scene has 8 animatronics in Splash. The art for Tiana shows 11 figures by comparison, an increase over Splash if accurate.
In another somewhat older piece of art for the Disneyland variant, there's what appears to be at least 5 figures that are likely to be AA's in the foreground, and 7 silhouettes of other animals in the background. Keep in mind that unlike WDW, Disneyland's version does not have walls separating the scenes. You can see a lot of the scenes and AA's through other scenes as you're floating along the flume. So personally, i'm inclined to believe that those background silhouettes are supposed to be animatronics, not just flat images painted or projected onto the walls. You can also spot the old hanging Splash possums overhead as well. The only thing is I'm not sure WHERE in the ride this art is supposed to be (if it's even supposed to be somewhere specific), possibly the scene where Brer Bear is trapped in a rope.
If any of this art is remotely accurate to the quantity of the animatronics, then it wouldn't make any sense for the finale to only have 6. Even if the riverboat is removed, it'll still be the largest room in the ride and will no doubt be festive in nature. It makes no logical sense why it would have such a piddling amount of figures if the rest of the scenes have so many.