Sheet flooring over existing weatherboards

Hi All,
Longtime reader of this forum and first time poster.
Im renovating an old california bungalow and due to variances in in existing floor levels and finishes i need to bring the floor levles up approx 20mm in some rooms.
Iam planning on doing this by sheeting over the existing floorboards with 19mm plywood.
Just a couple of questions:
1. Am i better to screw fix or nail fix and does this need to be through the boards to the joists?
2. Do i need to run the plywood at 90 degrees to the floorboards?
3. Do i need to provide glue between the two as well.

Any advice is much appreciated.