heatpumpfan

It is not clear why your heat pump cuts over at 40 degrees if it still has COP of 2 at 8 degrees. Presumably a cost-effectiveness calculation if gas is cheap in your area. If you had heat strips instead of gas, the "cut over" temperature would be much lower. If your unit is capable of operating at full capacity below 8F, it would likely only use resistive heating below 5F. For most climate zones, that is fine and your unit does not sound like one specifically deemed a "cold climate" heat pump, the which now regularly achieve full rated output down to -15F. Of course, it's only rarely that cold even in "cold climate" zones so operating down at a COP of 1 for these short durations is not a real problem. The impetus for systems designed for lower temperature operation is not really aimed at those with cheap gas but at those living where heat pumps were previously only feasible for heating if using much more expensive ground source systems. Achieving year-round heat with only the heat pump eliminates the need for a furnace so you have to include this missing gear and installation cost in your analysis.

Are you able to say more about that? "We will have local contractors" sounds great but in my area I don't think you will be able to find anyone to work with you. The local contractors and supply houses have closed ranks to fend off internet sales. I can't even buy sheet metal from the local supply houses without presenting an HVAC license. These folks are seeing their livelihoods threatened by a looming shift toward the "appliance" model (which I for one surely welcome) so they would see partnering to service a model like this as slitting their own throats.

You have an older system. New systems can provide 100% rated capacity well below freezing. Mine is -5F and not specifically a "cold climate" unit. The unit in OP seems to offer 100% nameplate capacity at 5F.

Sizing heat pumps by the sq. ft. is hairy reasoning. Load calculations take into account climate zone, insulation, building materials, orientation, ceiling height, etc. Any recommendation based on 2D area is sketchy. For replacement of existing professionally installed systems, going by the capacity of the installed unit is a better shortcut.

Refrigerant, pressure, vapor injection, and a highly "managed" vapor compression cycle.

"Cold climate" air source pumps have been available for years. Even my cheap MrCool Universal has 100% output down to -5F and it is not even really a "cold climate" heat pump.

>"backup heating strips"

Big resistive elements sitting in the air handler. Think big toaster oven coils that are used when the outdoor temperature drops too low for the heat pump to handle the load.