I'm not sure why the -240ASME and not the 180ASME, which is somewhat better matched at low fire to your actual heat load. Do you have a major gusher-shower that needs to run simultaneously?
The fact that it's cut up into 4 zones could be an issue for any boiler- you're probably better off with a Polaris tank-type HW heater. The min-fire output of the 240 ASME is about 19,000BTU/hr, and your smaller zones (maybe all of your zones) are individually less than 1/3 of the total output, probably under 1/4 of the output, and there isn't enough thermal mass in that type of radiation to keep it from excessive cycling. With a Polaris and a heat exchanger the thermal mass of the tank keeps it from short cycling no matter how small the sipping of zone calls are, yet it's plenty of burner & buffering mass for both hot water and heating needs.
With your radiation and micro-zoning any of the Navien combis would likely be seeing a dozen or more ignition cycles & flue purges per hour during the heating season, which is hard on the equipment, and bad for efficiency.
A ~7K heat load on a 900' basement sounds a bit on the high side at Seattle's outside design temperatures. is the foundation insulated, or is it a walk out, with a bunch of windows? (I have 1500' of basement and a design temp 20F colder than yours, but only a marginally higher heat load for the basement.) With R15 continous foam on the foundation walls (Or an inch of EPS between the concrete and an unfaced-batt insulated 2x4 wall, no interior vapor barrier) and an insulated & air-sealed band joist/foundation sill you'd probably be comfortable during the heating season even without the baseboard zone. But it takes on the order 100' of baseboard to deliver 15,000 BTU/hr @ 120F boiler output, and if you cut that length by half you'd be hitting the short cycling range for the Naviens.