Lax,
It sure is looking like your pre-heater is not working at all. If the engine doesn't burbble, stutter or exhibit any changes while cranking with the flame heater switch on, it seems to me that there is no heat in the air inlet manifold. If the engine cranking performance does change while the heater switch is on but the engine still does not start then the problem is somewhere else.
It only takes 3 things to light a fire: fuel, oxygen & ignition source. If any of these are missing then the heater will not fire. I have not done this but it seems to me that you could disconnect the small diameter high pressure line at the nozzle and turn on the switch. (The accessory switch should be on, I think, and fuel stop should be pulled out.) Fuel should flow out of the tubing. If no fuel, you have to back up to the low pressure side of the pump and see if there is a supply there. Keep backing up in the system and if there is a blockage you will find it. The small fuel filter in the supply line looks like a suspect. (I know you replaced the flame heater system but if your fuel was contaminated or there was rust in the tank, you could have a plugged or water logged filter by now.)
So, if there is pressurized fuel to the nozzle ( the nozzle could be plugged but it is new so not very likely) you have to verify that the spark plug is working. If it sparks the only item left is oxygen. Surely, the air filter has been checked. If not, it is worth a look to see if the filter is clean and clear. I think if it was clogged you would have trouble getting sufficient power from the engine in the summer.
When I use my flame heater I notice definite changes in the cranking characteristics of the engine. It tries to catch when the heater is on. Starting takes a couple of tries but the engine finally lights and it stumbles to life. Without the heater, the engine spins at the same speed and does not even try to start.
Sorry if this seems too basic but there are not too many parts of the flame heater system and it might be worth a detailed check.
Just my opinion of course.