In Oedipus Rex, is there anything Oedipus could have done differently that would have saved the kingdom? Oedipus takes many steps to save the...

I would qualify
your question.  You ask if Thebes is doomed from the beginning of the play, but it should be
noted that it is not Thebes that is doomed in the play; it is ratherwho is doomed from the
beginning of the play.  Beginning with this premise in mind, it should be recalled that, while
Oedipus (and his father Laius) heard what his fate would be before he actually brought it about,
the actions which actually caused his tragic fate were committed entirely of his own volition. 
Even while these actions resulted from a course of events which were meant to prevent their
occurrence--e.g. Oedipus fleeing from his adoptive father's house...