Yeah... If Nomura had cared about Kairi even back in those days, I think she would've been a party member in Hollow Bastion in its second visit in KH1. Maybe they decided they couldn't work two party members in the same world back then, but I think it would've been enough to have Beast in the first visit and Kairi in the second.
I remember someone had a retrospective on KH in the past that sort of examined little things about the games and they supposed (and I think they could be right) that KH2 in some ways feels like they had initially planned for most worlds to have two Disney character party members per world. For example, most of the first visit to the Coliseum you're with Auron while the 2nd visit you're with Hercules in the tournament. In Pride Lands, you're with Nala for most of the first visit and then with Simba in the second, etc. Perhaps Will Turner and Shang would've been party members in Port Royal and Land of Dragons. It makes sense actually for Will to be your party member while you're on the Interceptor in the 1st visit and then Jack to be the party member when you ride the Black Pearl for the 2nd Visit.