you need a "traditional" backup fuel for any kind of solid fuel heating. Solid fuel heating is considered a supplementary heat source (even if it's pretty much all you use) and a heat source that requires no attention is the primary heat source. Most homeowners' policies require something other than solid fuel heat to be present and be considered primary heat. Additionally, if you are away from home for an extended period, you will want to have a 100% maintenance free backup. Often, these two fuel sources can be combined in the same unit - most on the market today offer that ability. Thus there is no need to have two furnaces or boilers sitting beside each other and the associated costs that would create. With anthracite coal heat, the initial investment is usually far less and the payback much faster than most other "alternative" energy options. Anthracite coal heat is not for everyone, but, with the modern appliances on the market and the significant cost savings, it's an option that's worth looking into.

very cold climate, passive and active solar, 1200 sq. ft. house. only have propane available in our area, new construction, energy efficient everything possible.

I am curious as to why you say an air sourced heat pump is not a good choice for your climate. If you will have net metering then are you able to sell any unused electricity back to the utility company? If you can then powering you electric boiler is not free. It is hard to estimate, but 5 - 6 kW of solar panels is about what you will need to power the house without the electric boiler. If you post your propane and electric rate a calculation can be done to determine which will be more cost effective. If propane is cheaper, than you could also use it for cooking, drying clothes, and domestic hot water.

What size boiler do you currently have? The next smaller size is rated 80K BTU for heating which should be adequate for you house and climate. The domestic hot water heater for the smaller unit is 150K BTU. That should be adequate if you are taking one shower at a time. The combi units can modulate over a wide range. I am not a combi expert, but there seems to be little down size in over sizing the unit other than cost. Have you spoken to a Navien dealer?