Unless I missed something above, it seems that some have experienced their DWR coated down performing well for many (25-30) years, and none have found that it doesn’t last well. But suppose it only lasts 2-3 washings. Why not just re-DWR them with Nikwax’s wash-in product? I checked with Nikwax and they say that normal down treated with this is as water resistant as the Nikwax down that many clothes and bags are sold with.
If we want to learn more about performance, may I suggest a simple test? If any readers have down mitts (or socks/booties), why not see what happens if you apply Nikwax’s treatment to one of them, e.g. the right one, and leave the other untreated? Then weigh them dry, expose them to moisture, squeeze out as much water as you can, and weigh them again. Then let them sit (or better yet, hang, a better approximation of what they would be doing on your hand or lashed to your pack). And weigh them again every X minutes. Then (or alternately), do the same test, but after getting them wet, instead of letting them hang, put your hands in them. Continue to test the weight, but also note how warm/cold they are. Now repeat all of this applying moisture in other ways (e.g. spray vs. soak vs. holding them open over steam vs wearing them on a hot day).
Regarding the clumping after clearning concern, some (Nikwax?) recommend that after cleaning you put the garment in the drier (no heat I suppose) with a few tennis balls. No clue if anyone recommends tennis balls for untreated down. If not, then I guess there is a clumping issue and, hopefully, a solution. Though it sounds like none of the users of DWR down here reported any clumping.
Regarding the safety of the chemicals involved, any danger is sure to be a function of exposure. So if you are wary of the chemicals, use untreated down (and fabric without a DWR coating, etc.?) for a bag you will sleep in 100 times per year for 20 years. But for a garment you only take out of your pack for extreme situations, or only wear to and/from work/school, etc., there is much less exposure. And while none of us know exactly how such risks compare to all of the other risks we face, this is not one of the ones that is going to keep me up at night.
P.S. Among the manufacturer’s mentioned, Nunatek actually does offer DWR down as an option. One could also add Rab and Berghaus. Possibly one reason a fair number of super light/high-end products do not use it is that it adds weight. Nunatek says 5%. So 900 fill power down becomes ~850 fill power down. A parka with 15 oz weighs almost an ounce more. And suddenly you are not offering the lightest, sexiest product on the block, and it’s harder to charge high-end prices.