Shawn having to take one day off a week doesn't stop her doing extra hours on the other days to make up, which, as she doesn't actually work, shouldn't be a problem, and her athletic training should certainly give her the fitness level and stamina to cope with extra training, as opposed to some of the less fit, and older contestants.
The show did say right from the start that Melissa had had extensive ballet training, and I think it shows, as her technique - especially with the Latin dances - is very poor. Latin should be weight down into the floor through the ball of the foot (a good Latin dancer will wear out his/her shoes from the inside of the shoe out if the pressure is right)' Melissas' weight it all carried too high, and consequently her feet tend to slip all over the place, and seem surprisingly weak.
But what could the show do? If they had chosen someone with no dance experience at all, what they would have achieved in 2 weeks may well have looked so bad that she would virtually be guaranteed to be first out - and that wouldn't have been fair.
I think dwts and scd should do what they do for Dancing On Ice - and have a couple of replacements lined up, and who start training alongside the others. If someone drops out before the show begins, the replacements are ready to step in; if not, they are often given a chance to appear in the show proper the following year. Either way, they get paid very well!!!!!!!