Given that both the desuperheater and the geothermal both need backup heat, the best approach is to install a small high efficiency gas boiler with 2 zones, one as an indirect heater for the desuperheater (so it'll ensure that the hot water stays hot, but won't run if not needed) and the second zone as a hydronic loop in the air handler to provide backup for those cold nights in the winter that are below the design spec.
Of course, the heat input to your house is a function of heat loss, so if the house is very well insulated and sealed against air changes, regardless of how you heat you'll save energy.
TBH, since geothermal is reversible, in that it does both heating and cooling this makes it even more attractive and efficient especially in the summertime. It should be required in new construction where possible and indeed heavily subsidized to encourage it's adoption.
Geothermal water heating just barely works out in general because efficiency drops with higher temperature differentials. If you would accept you water at noticeably below body temperature it would be great (though I suspect this is ideal for bacteria growth) efficiency. If you want your water as warm as the minimum recommended temperature by efficiency experts you are already in questionable efficiency range. If you go hotter it gets worse.
Not sure if they were right or wrong, but my experience with a lots of these heating and cooling things is that the real world efficiencies are often different from theoretical values.