Emily, to really "wrap your head around this" you have to think about the nature, life cycle, and transmission of these parasites. You stated that your vet said the animals would have had to have acquired these parasites 6-12 months ago in order for the quantities to reach their current level. From an outsider's perspective, it is quite conceivable that this happened while they were with you, since you have had them most of that time. However, it is also quite conceivable that they acquired the little buggers before they came to you. Maybe it happened resultant to cross contamination from imports, via any number of methods

. It doesn't really matter how they got them... or if they had them for a year, or just a day before Ed shipped them, does it (excepting, of course, that since you were sold CBB, you should not have received anything but CBB - so if they were imports, that would be an issue)? The bottom line is at least one of your snakes has these parasites. It happens, and more commonly than people think.
I understand that you are upset & concerned about the appearance of parasites within your collection, but I think that things have been sensationalized a bit more than was necessary. Maybe its because I have been doing this for a long time, and have dealt with plenty of fresh imports, but I don't see it as being worth all this fuss. Yes, you were told that they were as clean as you will find...and maybe, just maybe, they were when he said it...or shortly before.
These were yearlings when you bought them, correct? What were their weights at the time of purchase? Was their any indication, even in retrospect, that they were not growing as expected?
Don't get me wrong - I suspect that, at the very least, the BP in question was likely harboring some, if not all, of those parasites when it was shipped to you. Maybe they all were. Unless Ed had fecals done on them just as he was packing them up for you, there is no way he can realistically claim it isn't possible. Then again...you can't realistically claim that it/they absolutely couldn't have acquired them in your care, either.
Good luck with the snakes - the Albon treatment gets old before it gets finished, lol..especially if you have to treat more than one animal.