Flooring Tile - Gutted bathroom to studs and subfloor

I started on this project in June 2003 when I bought my house. I have been doing a little work here and there and finally at the point where all the old stuff is gone out of the bathroom. My bathroom is a 1970's style master bath that is 8'x4'4". I pulled up the old tile, which was mortared to a 5mm(1/8") luan sheet. Then there was another 1/2" built up floor which had some dry rott areas that I removed. And underneath all of this was my original 1/2" subfloor which is undamaged.

Will I need to rebuild this subfloor with 1/2" plywood and then lay 1/4" of thinset, then 1/4" tile backerboard, then 1/4" thinset, and then finally my tiles?

Or is it fine to just get 1/2" durarock/tilebacker board, 1/4" thin set and then my tile?

I was told to thinset under the backer board due to the warranty on the product.

Warranty threats are just the stick used to beat folks over the head to get them to do what they should do any how. Tile and wood have very different expansion and contraction rates, especially in todays world. You need a medium between them to compensate for the difference or the tile won't stay there. This is the purpose of the backer board. The purpose of the thinset under it is to bed the board so it won't flex and damage the tile.

Are you sure the subfloor is only 1/2"? Most I see are 5/8" or 3/4". If it really is only 1/2" you must add more plywood for sure. Will you be using natural stone or ceramic?