I am not familiar with specs for Swimming Pool construction. Is there an accepted drainage grade on the decking/skirting? Is it poured to drain water toward or away from the Pool? If it can be poured to drain toward the pool the liner can be laid directly on the deck/skirt. The installation that I pictured had a slight poolward grade so I didn't need to make any modifications to alter the drainage grade. I don't know if that was intentionally poured that way or if it was a shoddy job. If you need to alter the existing deck/skirt grade I think any material that you feel comfortable using would work.
The mortaring that I referred to was for tying the rock together (if you don't trust your dry-stacking ability). The installation I pictured is entirely dry stacked.
PM me if this is not completely clear.