This thread showed up in my "what you missed" email...of course, every now and again
@Mandy Onderwater uses my name in vain
I processed horseradish 2 or 3 weeks ago here and wanted to come on board to encourage you. Being in the middle of Canada (we got snow last week and it looks like it's here to stay for the season...a bit earlier than typical), I'm not of much use to you for a source. That said, as a reminder, horseradish is an incredibly tough plant. Those roots you've come across may very well be perfectly viable and could hydrate and become happy campers. Apparently they do prefer sandier soil, but we've had no issues growing them in our silty clay soil. Of course, it does make digging and cleaning up the roots a bit more challenging.
Our patch started when the neighbour found some years (decades?) ago in their travels (not many folks growing today, but many old farmyards have plants) and offered us a plant or two. No matter how much I harvest, I never get it all, so the plants are self-perpetuating. Recently I've been working on getting another patch established at our acreage...thus far the plants are alive, but not thriving. We transplanted more out there this year, so hopefully things will pick up.
I've never been fussy about size of root...yes, the bigger roots are woodier, but when digging in clay you take what you can get. The small flexible roots I just wash well, but the bigger ones get peeled as they develop a darker outer covering. In our experience I've found the older woodier roots to be stronger flavoured...if that's a correct observation, perhaps that's why the store-bought stuff tends to be milder (aside from being creamed)...smaller roots are easier to process which could lend themselves well to commercial practice.
One thing I do need to do is remember to dig them and process them a bit earlier in the year so that I can either open more windows or even do some of the work outside so fewer tears are generated (and sprayed onto the inside of my glasses).
So, I'm not much help relative to your questions, but forgive me as it isn't often I have a willing audience to listen to my blathering on about horseradish.