You don't want someone using the nutrients in your soil, but not doing the correct things to make sure they are put back in to the soil for next year. 

Our bigger issue is that the leaking water separate my house's slabs and soil underneath it, a huge cavity was formed under my house.

If it's 2' or less, that water won't get into the home and simply flow underneath (remember that mobile homes are basically on stilts made of concrete blocks roughly 3' off the ground).3) Is the water projected to be "pooling" of "current" -- in other words, is the water just sitting there or would it be moving at a rapid pace that could wash things away?

The builder is DSLD homes, he is building this as part of a >200 building neighborhood, seems by all means reputable, but I am also learning that foundation failure usually has nothing to do with the quality of the construction and more about the underlying soil content. 

If the lowest grade can be raised to the BFE or higher without violating residence codes (most require soil be a certain distance away from your lowest floor to prevent moisture wicking, rot, critters, and termites), then you may be eligible to get an exception from FEMA called a LOMA - if you are successful the LOMA essentially states the property is out of the flood zone and you will get normal flood premiums.Summary: Government backed loans require the buyer to use FEMA premiums for flood zones, they will be high, and cannot be shopped through private flood insurance.

This stuff is underneath the tile, motor, and wire mesh in a bathroom that used to be an attic when the house was built in 81. 

I don’t think indoor pools will be a good idea unless it is some multi million dollar estate or those expensive deals in NY city.I had to remove the bottom of the pool to allow drainage and then back filled with approved soil with compaction reports and geologist oversight and stamped approvals.

Soil was analyzed and did not exceed CT''s limit of 500ppm.The seller has already refused to remove the oil tank.

Lastly, if the lots are not going to have city water/sewer and would need well & Septic then a soil scientist might be needed to tell you where a well/septic could go which might affect the lines the surveyor cuts.

I sold the property as is, didn't remediate the soil.