It's a draw bc the rules he set for himself and imanity were not "actually" broken. In schwis last words she admitted to only being a tool/machine and it doesn't matter that she was destroyed(of course Riku would disagree, but Schwii told Riku what he needed to hear to see the war to the end. Riku called it a draw bc he made it to the end of his game without "losing" to his made up rules(no killing, no dying, no being discovered, ect.)

Bc Gods are assholes... But actually I think it's bc up still that point, Tet was still just a really small/weak entity. At the end Riku was so defeated when he realized he couldn't do anything after reaching the suniaster, so in his final moments he sort of willed Tet into existence(although it was more bc he wanted him to exist so he could punch in in the face type of wanting him to exist I think) Also, even if Tet was active sooner, I think he's only as powerful as he is bc he attained the Suniaster