I disagree completely. I assume you meant "below zero" as below zero F and more than 17 below Centigrade ? If YES, continue reading what I am about to say. If NO forget my comments here. The symptoms you describe are typical of diesel fuel gelling. Not actually "freezing." It is guaranteed to happen at zero F and I have seen it at about 11 deg F. with an automotive diesel. The warmth from running it a short time, being inside a closed place no matter heated or not, probably loosened up the fuel flow. This fits perfectly with a case where you had added an additive recommended by the dealer (!) and you were right on the edge of more serious gelling. It is also a perfect illustration of why I claim that additives are only a marginal solution that help in a limited way and DO NOT fix the problem with any certainty. Additives are just a maybe of a fix. Most of us do not want a Maybe. I have said in many posts that the real and certain fix is to mix #1 fuel (kerosene) with your normal diesel #2 fuel in a 50-50 mix. I repeat one more time, this is exactly what the heavy equipment operators do in northern VT and NH and places with climates similar to yours. In the very cold weather, just run half kerosene. It will hurt absolutely nothing. The lube qualities are only slightly degraded and that for a relatively short time. Your power output will be reduced by a tiny % and you will NEVER notice either of those 2 side effects. By the way, your experiment essentially shows how much the additive lowered your gelling point (or rather how far it did NOT lower the gelling point.) Lowered it some, just not enough. The additives won't hurt anything but if you really want a reliable solution, mix in the kerosene. I do not have the numbers but ask your local heavy equipment operators -- they can tell you how far below zero F the 50-50 mix will cover you.
By the way, there is no reason at all to get into these messy (and potentially problem causing) screwing with filters. Once the engine is warmed up running on a usable mix of fuel for a while the gel will be "disolved" in the various filters. Only if it was so bad you could not get it to run at all would screwing with filters be warranted. Even then it would be better to just move it to a warm place for a few hours. Good luck and let us know how this all came out.