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I've been tempted to harvest roots to make D&B. Everything I've read says to get the roots of greater burdock (arctium lappa), I have a fair guess that there are one or two locally, as well as a lot of other docks. So the question is - is arctium lappa the only edible dock root, or are other docks edible as well?

You need to find someone who uses docks to make tea and coffee to answer that one. There are a few about.

It's definitely Arctium lappa you need for making D&B or coffee. I'm not sure other docks are the same genus, although some bits of them are edible. If you were genuinely starving you might consider grinding the seeds for flour but otherwise they're best kept for relieving nettles stings (and they're not even useful for that). Others may have other ideas.

I think in quantity, regular docks can be a bit toxic due to high levels of oxalic acid, even animals would rather not eat them, I have noticed my alpaca's will eat them when they have been pulled up and wilted for a week, maybe it makes them more palatable then