Impregnating sealers will not affect the COF of your stone (unless over applied).
As far as the underlayment, 1 1/4" is ideal (3/4 + 1/2). However, I have had success using 3/8" Halex (7ply birch plywood), and Schluter Ditra crack isolation membrane. The Schluter is basically an additional 1/8". You can also use Noble CIS which is basically a 40 mil membrane plus the thinset (or mastic) to attach it.
While other methods are available, the one I listed will keep you within industry standards, and keep your stone problem free from cracking (if correct joist spacing & lengths are followed).