Just finished replacing our two dead HVAC systems — a few observations

Dual-fuel relly is the sweet spot. We made a similar swap two falls ago, putting in a 3-ton Bosch inverter heat pump over a two-stage Rheem gas furnace. The biggest thing that made a difference for us was spending some time dialing in the thermostat changeover point. The installer set the gas cutoff way too high at 40 degrees, so we were barely letting the heat pump run before the furnace kicked on. Once I lowered that threshold down to 28 degrees on our Ecobee, our winter gas bills plummeted without losing any comfort. The steady, moderate heat from the pump actually keeps the rooms feeling more consistent than the aggressive blasts from the gas burners anyway. Good call on using the energy audit rebates, too. Getting that certified and signed off thru our electric co-op shaved about $2,200 off the invoice on equipment we were already planning to buy.

We almost went the dual-fuel route last year when replacing our 20-year-old Carrier split system, but we ended up cutting the gas line completly and installing a 4-ton Mitsubishi Hyper-Heat inverter instead. Our main motivation was the monthly customer charge. Our local gas utility charges nearly $38 a month just to have the meter hooked up, even in mid-summer when we weren't burning a single therm. Paying almost $450 a year in base fees just for the privilege of having gas backup didn't make financial sense once we priced out how well modern low-ambient equipment performs down to negative digits. We made it trough a -6°F snap in January without auxiliary heat strips ever engaging, and the registers blew warm air the entire week. If your ductwork is sized right and you have decent attic insulation, you don't always need to keep the fossil fuel connection around as a safety net.

Did the crew do a long nitrogen hold on that upstairs line set before brazing everthing in? You mentioned the old attic unit was completely flat on refrigerant—have you ruled out a pinhole leak in the copper itself? Flushing cleans out the old mineral oil, but if the original leak wasn't actually in the coil, reusing that hidden line set could mean you're slowly losing charge on the new heat pump right now. It might just be my paranoia, but I'd definitely keep a close eye on that upstairs unit's cooling performance once the warmer weather hits.

Make sure you check the blower DIP switch settings on those new furnaces to verify the CFM profile acutally matches what the heat pumps demand across both modes, because installers notoriously leave factory default settings in place that favor fossil-fuel temperture-rise curves rather than the nominal 350 to 400 CFM per ton required for proper refrigeration cycle heat transfer. If they didn't verify the total external static pressure with an inclined or dual-port digital manometer across the cased coil and that secondary heat exchanger, your blower coud be quietly over-amping and giving you degraded latent capacity once cooling season rolls around.