Inadequate Main Drain Slope Under Mono-Slab
I have a 5 year old home with mono-slab and about 92' of main line drain (sanitary sewer) under the slab measured by camera and cable from farthest toilet to clean-out just outside the MBR wall. The line is 3" PVC, so the minimum slope to meet code is 1/8" per foot. This works out to a minimum drop of 11-1/2" in 92 feet if the pipe were laid perfectly. Measurement of the difference in the pipe's depth at the clean-out and at the farthest toilet (removed) show that it was installed with just 6" of total drop, 5-1/2" shy of absolute minimums. The builder has gone through Chapter 11, but the plumber is still in business.
I've been flooded by a sewer backup (major damage to hardwood floors), and had several stoppages. A camera inspection shows a substantial belly under the MBR, and several areas of poor flow or reverse flow elsewhere when toilets are flushed.
What is the right way to fix this? I don't want a degraded home -- weakened slab, or compromised foundation, or noisy, unreliable pumps, or non-functioning sewer.