Re: Subfloor repair question
Gizmo --- from my recolection backer boards were to replace the old mortar and mesh system.
Tile over plywood can be a suitable system considering the modified mortars and additives available.
Floor structures --- unless spec'd --- are rarely suitable for tile without some modification.
With 12 inch centers joists are typical for tile spec'd floors and 16 inch centers can be used depending on span.
Besides i believe uncoupling membranes are a good idea for a substrate.
"" an ounce of perception -- a pound of obscure "