The reason that the there was a clone of the white ranger is because in the Japanese series Bakuryu Sentai Abaranger, which the action footage in this show is taken from, the white ranger is fighting against the others throughout most of the series, so the 'clone' was necessary in order to be able to reuse much of the footage.
Jason Lee Scott (Austin St. John) was originally considered to be the Black Dino Ranger, but, due to Tommy's popularity (and possibly because Jason had previously played a Black Ranger), Tommy was chosen instead.
According to Catherine Sutherland, there were plans for her to reprise her role as Kat, but this was scrapped because of the cost of flying her out to New Zealand. Presumably, she would have been Tommy's wife and likely filled Hayley's role.
Had Jason David Frank not returned for this season, the Black Ranger would have been named Edward Payne. Because little was changed outside of the name, this lead to the canon issue of Tommy becoming a Mad Scientist in seven years.