I guess I need to explain why fibers and based 30 years plus of doing pavement research I respectively believe fiber are better that rebar or wire mess for controlling cracking. First of all you need to make sure your base is solid. When we prepared the site last fall, we removed the organic soils and compacted the sandy loam soil to 95% max density. (I ran the tests) We then put 12 inches of MNDOT class 5 gravel in 3-inch lifts compacted to max density. Having read you post I believe some of your issues are not enough binder in gavel to supply strength. As far as not see any signs of fiber are you sure the supplier put them in and if they did, they did you check to see if they were uniformly distributed. Since i am data guy and have access to testing equipment I cast strength cylinders. To see how much strength, we gained using 3 lbs. of macro fiber. The concrete mix used was 4500 psi mix design without fibers the 7-day breaks were over 6000 psi for the cylinders. Normally 4500 psi concrete from this supplier would have 7-day strengths around 5000 psi.
How the issues I have with rebar.
1. to work properly the rebar must be place exactly 1/3 of the way from bottom of slab. Any higher you risk spalling of the concrete and lower you do not get value for reducing crack. What you're trying to do with steel or fibers is to control micro cracks from forming. The micro cracks if not sealed will allow water in which will case issues with marginally constructed base and spalling of the concrete. IMO the only way to guarantee placement is to use chairs to put the rebar on. Per specification the maximum chair placement is 2 feet apart. The unknown about placing rebar on chairs is did any of them get knocked over while placing the concrete. The rebar being supported above the base is a trip hazard also.
2 being we placed tubing for in floor heat I did not want to risk low steel damaging the tubing.
3 If you place rebar in concrete that is rusty you risk pack rust from forming. This can be a issue if the PH of you aggregate is wrong.
As far as long-term performance. MNDOT has use fiber for over twenty years on Interstate highways. The areas they use them are over what they call trench support areas or high-water table areas. Before fibers they use to build rebar mats out #12 rebars on 1-foot gride supported on chairs. The oldest one where they replace the rebar mat with fibers was place across a swamp I35 W just south of Minneapolis in 1998 and the pavement rides good and has no middle panel cracks.
I would leave you all with this thought, use what every you comfortable with, there is may ways to build a good concrete pad.
Last point the Romans showed us how to build roads. The 2 key points they left are; good drainage and proper density are crucial.
Have a safe and Happy 4th.