Tile floor installation on top of hardwood

Not doable at all.

Old houses have 3/4'' thick wood planks as subfloor, not 8x4 sheets of plywood like today's houses, so I would say that it's ok to consider your hardwood floor the subfloor. That's if you're putting another wood floor on top.

However, for the purpose of tiling you need an additional layer of plywood to reduce deflection that will cause the grout to crack eventually. And on top of that you would install your cement board. The cement board itself provides no structural support and is only used as a decoupling between a wood subfloor and the tiles, so the lateral movement of the wood is not going to cause the tiles to pull up. You can always use a durock membrane to reduce thickness, but you still need a floor that will not deflect.

If you really want to do this right you would take out the old floor down to the joist and install some OSB and then do what you want...

Unless you are concerned about an asbestos material under the tile (such as old linoleum squares), I would remove any old material and start clean. If you keep building on top of the old materials, you might need to rip all your doors down to make them fit with the new floor. Conversely, you need to consider how high to make the new floor in relationship to the old floor. Last, while you may still need floor leveler, a better bet is to install a nice new clean level underlayment board such as concrete board, durarock, hardibacker, or other tile board for tile or a nice clean 1/4" sheet of plywood and paper for hardwood.