Hey, you gave it a good try and that last post was gold, but that propane leak has you scared and I don't blame you for not wanting to pursue this any farther. I blew my eyebrows off and altered my hairline playing with an recalcitrant RV furnace once.
But I think you're really close to solving this. If it was me, I'd mix up a soap solution and find that leak. Then I'd take an air hose to that furnace and blow the s**t out of it. I think you had a sticky sail switch caused by spiders or dust.
Good luck.