Let me try to answer your questions as below including the question raised to me in QQH.
1)Let us see In case the drainage is not good. Whether or not the rain will intrude into the subgrade depends on how heavy the rain will be. Once the total weight of your house is less than the buoyancy (just assume the area of the house become a pond, and then it will have buoyancy for anything in the pond), and there is no rain intruding into the subgrade. Otherwise, the rain will intrude into the subgrade (less chance for this situation). If this situation really happens, then within 33% compression of the rigid insulation will cancel part or all (depends water contents in the subgrade) deformation due to frost heave in the winter, and the rest of deformation will be transferred to foundation wall, if this force is big enough, then the foundation wall will be bent somewhere, which we can call uneven settlement.
The gravel underneath of footing wall will reduce the accumulation of rain, but putting gravel underneath of all footing wall will be much more expensive, I personally do not think the developer will do it.
2) The PDF file you have shown me does not include the geotechnical report, the two reports included are BIOLOGICAL Report and Geotechnical slope report. Whether or not the subgrade can take the upper load will be specified in the geotechnical report as bearing capacity in the suggested soil layer. But you can do your own calculations for the total load on the subgrade based on 20lbs/sf on roof, 40lbs/sf on floor, 10lbs/sf on siding, it will give you rough idea how heavy your building is, I am pretty sure, it will be less than 50kpa(5ton/sqm).
Good luck