Engine shakes BADLY!!

Are you absolutely 100% sure that the firing order is correct. I know you went over this already, a long time ago, but.............
One time I had a bad shake on a 4-banger,that turned out to be a cracked sparkplug porcelain insulator.It was all but impossible to see, and sparked perfectly in open air. I swapped the plug around to a different cylinder and saw the problem follow the plug. Bingo! Under the magnifier, I spotted what I thought was a very thin carbon track, but was in fact a crack.
I suppose you have not been able to source the shake to a particular cylinder? In which case a leakdown test, might at least rule out multi-valve leakage.