Rob,
1st, you asked about my Collins. In general, I'm a Bosch guy when it comes to jiggy's. But I use the barrel Trion as my dedicated coping saw. I find that it's blade guides seem to help keep the blade straight when cutting curves and back-cuts in crown. If you, or anyone else, go this route, be prepared to re-work the Collins foot a bit to fit the Trion.
I needed to cut the foot, where the back of the blade hits, back about an 1/8 - 3/16" or so. When I first installed the foot, I was breaking blades like a newbie. I then installed a metal blade backwards, ran the saw, and applied pressure to cut into the foot. Worked for me. It may have been just this particular foot, saw, or operator. In any case all is fine now, and I do like the Trion with the Collins. ( I have also used a Bosch barrel with a Collins foot, works well)
As to crown "flats". When nesting crown, I've found that the flats don't matter as much as keeping the stops consistantly in the same position. "Forcing" if you will, consistant cuts through-out the job. If the flats are out so much as to cause problems with cuts, it would seem to me that the other milled surfaces on the trim would also be out, causing at least as much, if not more, of a problem.
Just some thoughts, Dan