When Felix meets playwright Neil Simon, he's essentially breaking the fourth wall since Mr. Simon is the author of the play The Odd Couple, and Felix is a fictional character from the play.
Penny Marshall appeared in 1974 in 14 of 15 episodes of the Paul Sands Friends and Lovers show which began in after she left the odd couple and before she did laverne and shirley in 1976.
This is Penny Marshall's final episode as Myrna, but the viewers do not see Myrna quit her job. Her last scene in fact has her merely leaving the office with Oscar to meet Felix, who invited them both out for lunch.
Miriam, Felix's girlfriend, is mentioned but not seen in this episode. In fact, once Felix begins going to the theater regularly, we see a series of women Felix takes as dates.
Myrna (Penny Marshall) quits (as Oscar's secretary, that is) and leaves the show. This was done so she could start work on her brother's new series; a little show called Laverne & Shirley (1976).