1. The copyright in Mario & Luigi for Geno says Square Enix. Not "partially xowned by Nintendo and Square Enix", Nintendo isn't mentioned as having ANY right to Geno's usage.
2. Okay then. Sakurai has literally no ability to just use whomever he pleases from third party candidates. The way you're acting is as if Sakurai has the rights to every character ever made.
3. Rumored DS sequel? Huh. And regardless, you're ignoring two points: First that Sakurai doesn't necessarily want to put Geno in, and second, SE doesn't necessarily want to. If they were to put anyone in, why wouldn't it be to promote a major franchise like Mana or DQ?
Your arguements are nothing but assumptions...
Square is apparently making a sequel to Mario RPG that works like Kingdom Hearts, as in Final Fantasy character interacting with Mario characters. Or so it's rumored in magazines and such...
Sakurai already showed interest in Geno and has talked about him, also he has 4-5 mentions on the poll...which is PURE Sakurai bias.
3rd, Sakurai chooses the characters for Brawl, not Square...He can ask Square for their permission to use a character and charge, but they aren't going to force him to use another character, and the chance of them denying Geno is very poor.
Also, Sakurai doesn't really care about Promoting Square-Enix's franchises and wouldn't pick one just to...Square promoting would only happen if THEY approached him...which they didn't. They weren't going to as said in an interview with Famitsu.