Couldn't tell you, re: Banjo.
As for believing the Ken leak? Yes, I do. COULD it be fake? Certainly. However, look at the sheer number of details the image got right.
Feet clip through sloped floors. The shadows are very close to identical in the 're-creation from the demo' image versus what the leak image had. UI behavior is plausible and relatively in line with other Smash games. Ken being in as a Ryu echo is entirely sensible in terms of fan appeal, amount of developer effort required, etc.
Thus we are left with two scenarios.
1: The image is fake, but the person took the time to research a plausible candidate, research feet sloping through floors, got a clean image of Ivysaur to work with despite the fact most people only have access to public-event demo builds (direct capture footage exists, but not all THAT much of it), researched how shadows and lighting worked and got them all correct (or very close to correct) for 4 different characters, faked a developer-build user interface on the side, faked changes to UI icons, and did it all on a stage that is new to Smash Ultimate. Then he got it to look like a magazine article's photo. Convincingly. The amount of time and skill required for this is tremendous, for relatively little pay-off other than maybe impressing potential employers.
OR
2: Everything matches up as well as it does because it is real.
I'm not saying scenario 1 is impossible. I am saying the amount of evidence favoring scenario 2 is so high that it passes a reasonable threshold, and believing scenario 2 requires far, far fewer assumptions than believing scenario 1 does. So, yes, I believe Ken is in Smash and have sound reasoning for it.