If you have a digi camera perhaps you might be able to take a couple of pictures [folded, unfolded - couple of closeups maybe) of how it's set up and post them here on this thread? (or your video maybe?)
It's hard to offer possible solutions to the problem without a visual reference...
I feel your woe about the costly exercise of returning it so perhaps some of the people here might be able to spot something amiss from the pictures that a non-brompton mechanic might not have spotted.
Sorry it feels like a waste of money but it's surely only teething trouble and I assure you once it's running properly it won't. These are not badly made or faddish bikes -they are the results of many years of design evolution so it's pretty likely that at worst - you will need a new tensioner but at best it might just be a case of moving or wiggling something...
Huw