What is the best way to bridge a huge gap between hardwood and tile?
Updated based on revisions to the question.
The way I see it you have two options, neither of which is nearly as complicated as your proposed solution.

Install a tile accent row with a beveled or bullnose edge or reducing edger (metal trim). This could be from leftover original tile or something else. 1/2&quot; cement board on the subfloor boards will be enough of a filler to bring the new tile level with the existing. Grout to the existing tile.


____  _______________
____||_______________\ ________________
____||________________|__________________
 ^             ^             ^
existing   new backer    existing wood
  tile      &amp; tile           floor


Install a hardwood strip with a beveled edge, stained to match the existing floor. This could be ripped from a 1x6, then set in construction adhesive and attached with a few finish nails. Optionally rabbet a small distance to overlay the existing wood floor, creating a clean transition line and covering any rough edges. Keep this minimal to keep it robust. I picture it like so:


____  __________________
____||                __\________________
____||________________|__________________
 ^             ^             ^
existing    transition    existing wood
  tile        strip         floor

If you have more specific questions about either approach, go ahead and ask.