CDR & Pilot Illness

With all the talk about the Misson Specialist being sick -

Would the shuttle be able to make it back to earth
if either the CDR or Pilot being incapacitated ?
Of course it would be able to if nothing untoward would happned
but what if some event happen that required human input?
Are any of the other astronaunts on board trained (somewhat) to sit in the
two front seats ?