I spoke with Western Mountaineering about it and trust them. My 30+ year old down items still loft to the MFG specs, so I'll keep going with what the OEM says.
I'm also a materials engineer and have zero concerns over the resilliancy of the protein that a feather is made from. Creases would actually increase dimension if it were even a concern. Even if that did happen, all the hydrogen bonds that give it shape would reset when you get it wet. The hair on your head doesn't take a permanent shape from wearing a hat. The only thing that's going to reduce dimension of a feather permnently is if pieces break off, which is really only going to happen if you denature the material with an organic solvent or some similar chemical.
I don't know of any companies other than WM that have done any sort of empirical testing on down that is scientifically repeatable, especially long term testing. None of them are hiring materials engineers, so unless they are doing in-house testing that they aren't publishing (which would be a dumb marketing move), they are simply going off anecdotal evidence and whatever their materials salesman tells them. Good example of this is hydrophobic down treatments.
Your REI bag probably just needed to be washed. I've bought old mildewy bags off eBay that more than doubled in loft simply with a good hand wash with a down specific detergent. I hand wash them because most front loaders won't get the bag saturated until the spin cycle. The bag will just float on top of the water.
In all seriousness, I've never noticed any degradation in my synthetic or down coats, whether hung from a wire hanger or a wide wooden one. I just don't think it matters too much. But I do tend to hang the more expensive gear off the nicer, wider, wooden hangers.