Question
I am preparing holes to plant new 2' tall Leyland Cypress.  At my last home I prepared the holes for 18 trees by digging a 36" diameter hole 15" deep, followed by a 18" diameter hole 20" deep with a 8" hole in the middle that was 12" deep.  I back filled the hole with a good mixture of top soil and compost and soil from the hole.  At the bottom of the hole I placed a evergreen fertilizer spike, and added spikes alone the perimeter of the larger holes.  As I back filled I packed the soil the best I could.  The soil had just enough moisture to pack without forming a hard layer.  After preparing all 18 holes, then prepared the planting holes with twice the diameter of the root ball and the same depth.  The soil I used did not have any Fertilizer except a root stimulator.  I covered the 36" diameter soil with mulch.  The soil (12") around the trees was higher than the outer ring with a small bank of soil around the 36" hole.

When be moved, the trees had grown a height of 18 to 20 feet. They also withstood some very high winds.  My wife said she could almost see the trees growing.  We had no problems with disease or insects.   After the branches grew pass the 36" ring, I put in additions spikes about 18" deep at the drip line of the trees.

Our neighborhood is getting ready to plant trees behind our homes next to a highway.  Everyone tell me the preparing the hole as above is not the prober way and we may loose the trees.  I agreed to check on the internet with an expert to get guidance.

Answer
Your method will work it maybe a little over kill on the size of the hole but the concept is sound except I would not use the tree spikes. Fertilizer spikes are concentrated fertilizer and if a root comes in contact with the spike it tends to burn the root.

You can get by with digging the hole for the trees 2-3 times the width of the rootball and as deep and fill the hole with good top soil or potting soil. Mulch with not more than 3 inches deep of organic mulch not piled up on the trunk. You can add some 10-10-10 fertilizer mixed into the soil but not more than a cup per hole.

Your method will give the root system more non-compacted soil to grow in and if you want to go with that much work it is OK. They will grow faster that way usually.  But at the minimum use the 2-3 times the size hole as above.