Welcome to The Community!!
Apply Leveling Sand and tamp it smooth under the first coarse of wall stone. This helps to level that first and most critical step of the installation.
Use an angular stone such as Vigoro All-Purpose Stone to backfill. Smooth stones like River Rock will not lock into place as well. Tamp it until it is firm.
Adhesive will not be necessary when using this type of wall block. If you add caps to the wall, Landscape Wall adhesive can be applied to secure the caps.
I see no issues with your design. You'll enjoy your wall for many years!
Thank you for asking,
Leveling Sand should be used under the Wall Stone. Sand yields a smooth, compact base for wall and patio stones.
Flex Base is not necessary unless there is going to be a substantial load on the stone, such as driveways and taller walls. Load will not need to be considered with a two foot high wall.
Do not use sand under the back fill stone. Angular stone, like gravel is best for that application.