There is a scenario that could unite team real and team insider and that is, hear me out, the banner is incomplete.
Yes, I know it sounds absolutely crazy but I also think its plausible for a few reasons.
1. Shadow, and Ken are echos so likely didn't require a ton of development time. It's possible Chorus Kids use recycled assets from Smash 4 that never got to see the light of day. Isaac is basically a recycled assist trophy, so he also didn't take as much development time as a normal character. Altogether these characters were made with greatly reduced development time.
2. The banner could still very much be a work in progress (with the creator still messing about with character positioning, for example, in the leak, part of Wii fit trainer's face is blocked. This is a big no no (i'm not a graphic designer but I know better than to block someones face in a picture) this implies that there is still adjustments to be made maybe in anticipation for...
3. Incineroar. The biggest contradiction between the two camps is that there is "no Incineroar" I think it's entirely possible that Incineroar simply hasn't been included in the banner yet because, again, the leak was the "work in progress" version.
I'm sure insiders have some quality information, but things are bound to change during development (hell plans change weekly at my work and we aren't doing anything nearly as complicated as developing a videogame) so who know's whose information is still completely 100% up to date. At the end of the day, we can all just get along and wait for things to blow over.