You don't need to tile the whole floor nor do you need the hardibacker under the shower pan. You do need to verify the floor is sturdy enough for a tile installation. This includes the joist structure and the subfloor. You mentioned adding ply, this is normally good. What type, and how? Also, make sure to follow the instructions on the hardi...it needs to be bedded in thinset and screwed or nailed down per the specs, then (you can do this while tiling) you must use the alkali resistant reinforcement tape on the seams.
A good resource to check out things for tiling is www.johnbridge.com. There is a deflection calculator available in the forum that will verify your joists are sufficient. No sense in doing all that work and have it fail. Note, from what I read, the simple failure of not using thinset under the cbu often doesn't show up for as much as 10-years. Not taping the joints maybe a year or two.