Is there one that seemed to be the most competent and knowledgeable about it? You might want to call them, explain your situation, and ask for a more thorough review of you HVAC system and need for changes. Sometimes they just can't figure it out on the first try, and the second time they can be prepared to spend more time and effort figuring it out. They know it will be a challenge, and might have fresh ideas.
But I totally understand your frustration, I have been there, it's a pain. But don't do half measures when your gut is telling you it could be something major. The extra heating costs, the uneven heating, the lack of air flow, all points to some major problem in the system. I would keep focused on a bigger fix even though very frustrating to keep pushing the experts and experimenting. All it takes is for one expert to finally find the fatal flaw and it would all be worth it.
I would start by sealing the hottest room (both intake and outflow) with cardboard. Try to do whatever it takes to shut down every form of air movement and force the cold rooms to get air circulation. If air is leaking through the roof, then closing upstairs door and vents and sealing the doors with a towel should stop that upward flow, and you can then figure out a permanent fix if that helps.
Edit: Also, don't forget that sealing the crawl space will have a huge effect in preventing upward air flow from leaking out the roof. Any air leakage at the top must come from somewhere, so the tighter it is below the less force on the leaks above.