Well then, a good majority of the information was already provided by Damiani in the discussion you both had and then followed up on Twitter, but I'll give it my fair shake.
For starters, five star characters are not necessary at all to succeed in the game nor are they heavily encouraged to the point where monetary investment is necessary in order to proceed. The feather cost is initially a problem to upgrade four star characters to five star, but it seems this current implementation is accounting for long-term investment rather than immediate reward. Nevertheless, Nintendo has been handing out feathers in a more frequent matter now with recent promotions rewarding 10,000, 1,000, and another 1,000 respectively without the requirement of ranking high in the arena.
Secondly, Damiani didn't find a secret well of experience in order to conquer chapter nine in one playthrough. The game was initially advertised to feature a training tower that allows you to level up your old and newly obtained characters without facing the same stamina requirements as the main missions. In addition to that, there's currently an incentive to utilize the training tower as the stamina costs are cut down even further to the point where you can greatly level up with just your initial allotment of 50 stamina.
If that wasn't enough, Nintendo has also been providing a healthy amount of free stamina potions during this initial opening period. It's possible we'll see lowered frequency as the game goes on, but stamina, so far, hasn't been a large problem unless you're looking to complete everything possible within a couple of days.
One last tidbit is that My Nintendo is also offering Stamina Potions, Duel Crests, and other various upgrade items at a relatively fair point cost for those folks who would like to gain an extra advantage without investing money into the game.
