While I totally understand your points, one thing I'd like to point out is the fact that Sakurai doesn't seem very shy about throwing his corporate Nintendo overlords under the bus if he disagrees with their decisions. That said, he's never mentioned them as the culprits behind Geno's exclusion thus far. This makes me very suspicious as to what's been going on behind the curtain here.
As for the marketing thing, consider this: What possible marketing strategy could there have been with regards to say, Piranha Plant? I know this topic is fairly controversial, but let's look at the reasoning here...
1) Mario Franchise: Oooooh look another Mario character! That should drive up sales, right?....Uh really though? The Mario franchise already has the most reps in Smash with the only competition being Pokemon and soon be Fire Emblem. Does anyone honestly think that adding a generic Mario enemy of all characters is going to significantly impact the sales of Smash Ultimate? C'mon, people. COME ON. Why not Waluigi? Why not a myriad of actually NAMED Mario characters? Even if we got any of them, would it make a significant impact anywhere near the third party characters? Probably not.
2) Iconicness: I love it when people throw this around as a justification. "Oh, but the plant is iconic and highly recognizable!" Yeah, sure, but so is Goomba...or Koopa Troopa....or hammer bro...or Kamek/magikoopa, the latter three having actual LIMBS to fight with being a plus. If we wanted to go purely based on "iconicness" why not pick hardcore mainstays like them? It just feels so arbitrary.
3) First Party vs Third party: I think it's pretty safe to say that third party newcomers have always had a bigger impact on the appeal of Smash for one simple reason: Crossover hype. That's the ENTIRE DRAW of Smash bros in the first place. People want to see Mario vs Sonic, Snake vs Cloud, Megaman vs Banjo, crazy off-the-wall grudge matches you'd never see anywhere else. If I wanted to see Mario beat up a Mario enemy, I'd just play....(wait for it)...a Mario game. Granted, I know third parties are probably expensive and harder to procure, but I would imagine that their impact on sales is worth it considering that the amount of third parties has continued to skyrocket since Brawl. Makes sense, since each one expands the ludicrous reach of this particular crossover. MORE CROSSING OVER. So why lean on the mundane like a generic first party enemy?
Mind you, I'm not so much trying to bash Plant or debate its place on the roster as much as demonstrating that there doesn't seem to be much logic or intelligence to Nintendo's marketing strategy when it came to picks like that if they did in fact push their inclusion. To be utterly frank, if the choice was indeed purely creative on the part of Sakurai, then I have to say I find his judgment somewhat lacking at times. (this is just my opinion, please don't lynch me) I'll tell you this though, if we get a different SE character instead of Geno, it will say VOLUMES as to Nintendo and SE's feelings towards Geno.