Floor tile installation

[Karen O.]

I know tile is to be installed over cement board.  The plan was to install 1/4" durock over the 5/8 plywood subfloor. 

My laminate floor installer took initiative and installed a marble threshold (he was installing laminate in hall) before 1/4" could go down in the bathroom.  The threshold is not coming out without being destroyed. 

Installing 1/4" board would cause porcelain tile to be higher than threshold.

The subfloor is very solid. Would adding a wood preserver before floating a thin layer of floor leveler be a viable alternative to the 1/4" c-board?

I fear the threshold has to come out because he didn't account for the 1/4" durock or the mortar. Or the subfloor has to come out and be replaced with 1/2 durock.

Thanks in advance for your suggestions.