Several years ago I built a 3 foot high x 24 foot long retaining wall
with a stair case in the middle. This required me to dig 11" inches
down for 6" of base plus 5" of the wall to be under ground.
Compacting the 6" of base in the channel was back breaking because
backing up the compactor and attempting to turn the compactor 5" below
ground was nearly impossibe.

For a wall this high made of 5" High x 12" Wide x 9" Deep block, would
hand tampering be OK?

Also, can leveling sand be used for a wall this high? I got the other
wall straight as an arrow last time, but it took a full day just to
lay the first course down.