Matt McCutchen
Hi Elliott, thanks for looking into this.  I guess I neglected to say, I strongly suspect the tag should be "turn:lanes:backward=through|right", though I don't know if it's good practice for me to make such apparent data consistency fixes without re-surveying the original data (I'd appreciate any advice on this side topic).

Here's a screenshot of "turn:lanes:backward=through|right" in the turn lane editor: https://mattmccutchen.net/private/osm-note-2692318-turn-lane-editor.png .  Comparing the two screenshots, I struggle to see how you could reach the conclusion that "turn:lanes:backward=right|through" is more likely to be correct.  In your screenshot, we can see that the selected turn icon for backward "lane 1" is pointing across backward "lane 2", so a vehicle in "lane 1" could collide with a vehicle in "lane 2" if it tried to execute that turn.  In mine, "lane 2" is turning away from "lane 1", presenting no collision.

I think it's implausible that the "potential collision" layout is the one that is actually signed; if it is, IMO that is bizarre enough that the way should have a "note" tag or something similar confirming it.  If we're concerned about this possibility, it could be settled by a re-survey.  If we accept my assumption, then the remaining question is whether the "turn:lanes:backward=right|through" tag could possibly have a valid interpretation other than "potential collision".  I think the answer is "no"; I didn't find any written documentation that was totally unequivocal on this point, but I think the fact that taginfo shows 10396 occurrences of "turn:lanes:backward=through|right" compared to only 49 of "turn:lanes:backward=right|through" is pretty persuasive.  But if needed, I could ask for clarification on some OSM forum.