For this type of wall typically the only footing required is 6" of #57 stone compacted and leveled.

I would wait for 2-3 months before making the purchase so you can use the renters money to pay for the entire shipment and analyze their spending trends and see what items they enjoy the most and best prices that would out compete those places. convenience things like toilet paper, tooth paste, tooth brushes, laundry detergent, soap, bounce could be an option, and then you have snacks, cold drinks, phone chargers, etc.

maybe a toothbrush like one of mine did this month

However, you don't need to worry about compaction.

(the engineer is asking if there was a fill/compaction report) that makes me the most nervous because how do i even fix that?  

Not only is the sewer deeper but it must be placed at an Exact sloped grade, with thoroughly compacted fill/stone underneath to prevent settling.

Or replace it with a new one, and properly compact the soil around it.  

Or replace it with a new one, and properly compact the soil around it.  

The letter includes a timeline of communications with my contractor stating that he contacted them to let them know the asphalt was on its way to the site and was told at that time that permits had not yet been issued.I was told that while it was the County's right to order me to rip out the driveway, they were willing to take core samples to assess the compactness and depth of it's base, and have a engineering assessment conducted at my expense and, pending the outcome, possibly issue a retroactive permit.  

It now seems that the perimeter of the building is stable due to the piers, however there was some sort of plumbing leak under the slab in the one downstairs units, which washed out some more soil, and the slab collapsed by as much as a foot, rendering the unit inhabitable.There is a quote on hand for $22k from a foundation repair company to take out the collapsed slab, add some new soil, compact and pour new slab (all of which they supposedly can do without vacating the other 3 units), and another quote for about $130k to redo the inside of the affected unit, and correct other inside and outside cracks around the building and the other units.