What was Sanka's qualification to be an Olympian, again? He's really not much of an athlete. Derice picks him for the team due to his experience as a pushcart driver (dubious to start with), but then he ends up being the brakeman, not the driver.
What do you mean "qualification"? It's not like he had to beat out a ton of people to make it on Jamaica's team.
He showed up. That's qualification enough.
And watching a pushcart race is what gave the real team's coach the idea for a bobsled team in the first place.
Why would Junior's dad tell him to give up this foolishness and come home after they had already qualified?
Because he doesn't care. Remember how he dismissed Junior's protests that "the team needs me"?
And in the movie version, they were still seen as a joke at the time.