Pretty sure we had a whole thread explaining why the cast of FFVIII works as supporting characters to Squall, so I'm just going to ignore that part of your post.

This would be hard to do since you pick who you choose to go to the Missile Base with the exception of Selphie, so they would have had to design every scene thereafter with each character in mind depending on who you chose. I'm almost certain Rinoa would automatically go with you to Balamb if this was the plan from the beginning so right away you'd know something's up. I wonder how many people would've complained it was a cheap Aerith death redo if they did this.