The rule interaction says that a unit possessing a mortar is using Indirect Fire so no HA (C.1)
Um ... no. C.1 says nothing about "the unit possessing the MTR". (I'm assuming your quote in the post above is accurate as I am NRBH.)
and thus moving units are in OG for Concealment Loss/Gain.
And again, no. You are assuming (incorrectly) that if you don't qualify for the +1 TEM for HA that therefore "you must be in OG". Where do you derive that equivalence? Not even the relevant rules for MTRs say anything about "the target is therefore in OG"; the most that can be said is that you don't always get the +1 TEM for HA even when the firer is at a lower level. The rules for when you are and when you are not in OG are very clearly spelled out in the interdiction rules (and oddly, it's the one rule you have neglected to quote, perhaps because it renders your "thought-provoking discussion" rather moot).
If the mortar is broken, then the EXACT SAME unit would be using Direct Fire so HA would apply. IMO, the target does have HA in both instances. The rules don't support that. To me, that's silly.
What's silly is that you are reading "apple pie" and thinking that it means "steak". While they are both tasty food items they are not the same thing, and should not be treated as if they are.
FWIW, B1.14 also refers us to A10.531 Open Ground definition when referring to Hill/HA interaction so Advancing into an HA in the LOS of a unit possessing a mortar would also be Concealment Loss
And again, no. "Not qualifying for a +1 TEM vs that weapon" is not the same as "I am advancing in Open Ground". When I am being attacked by a FT I don't qualify for any sort of TEM, and yet if I am sitting in a building, I am sitting in a building and not treated as being in OG. HA is no different; it's a TEM that happens to be conditional.
The Q&A Klas quotes directly supports this. I'm not sure why he thinks some errata might be required.