I'd have expected one to every other tile at least. Our slates all have 2 each and still rattle when it gets blowy. But then at 1400' and exposed it does get a might windier than most places...
No, the water barrier needs to go on top of the insulation and all the timber in the drafty gap under the tiles.
Personally if you want a warm roof with the insulation on the outside of the rafters you really need to be getting proper advice from a roofing specialist. The normal way for a warm roof is the outer layer to be traditional with insulation between the rafters.