Quite frankly I am lost at what you are trying to say.
I fully agree that an upper level rubble location is part of the building - that is what I have been saying from the beginning.
And I fully disagree, since the rubble location is not a building location, it has ceased to be part of the "building" definition. It is still a non-subterranean location in the building hex, but it isn't part of the building anymore. (It's the shattered remnants of the level it's on and any that were above it, but it isn't part of the building.)
That is why when the rules state "in a building" with no disclaimer to either Location or Hex, those rules should apply to the upper level rubble since it is part of the building. (It is also in the Building Hex, but is not a Building Location).
Which then gets back to the fact that the C2.7 and B23.423 rules specifically state "building" not "Building Location" or "Building Hex" when they prohibit Guns from being in the upper levels (with exceptions).
The B23.423 rule against it says "upper level of a building" for occupation, and "non-rooftop building Location" for firing. Not a building location, so neither of these apply for the rubble.
The C2.7 rule begins with "upper building level" for the general restriction of no guns at all (with EXC: for rooftop mortars and fortified locations). Again rubble is not a building location so it's not an upper building level. Now if it said "upper level of a building hex" you would have something.
Then, C2.7 goes on to allow some sizes of guns in "building/rubble hex". First issue here is that it doesn't speak to levels, only hexes. The hex type we have been discussing is a building hex (it might become a rubble hex if the rubble falls to ground level). Either way the types of mortars we are talking about are permitted to be in a "building/rubble hex" under this part of the C2.7 rule.
C2.7 has no restrictions on fire from those locations/hexes, just about setup there.
There are three terms used in the rules, and they are not freely interchangeable.
Building - A generalization, there will be one or more building locations, in one or more building hexes making up the whole of a single building. Also covers multiple types of buildings such as huts, factories, stone and wooden, row houses, lumberyards, split-levels, and marketplace.
Building Location - One specific location that is part of a building.
Building Hex - Any hex containing a building. Other terrain types and locations can exist in a building hex, such as upper level rubble, sewer, combo with woods, road, etc. The presence or absence of those other terrain types and locations means nothing to the status of the building, or those other terrain types because of the building. (Other than combined effects such as MF/MP cost.)
The marketplace is a great example of a simple instance where multiple terrain types coexist in a building hex. The OG location at ground level doesn't bend anyone's mind, why is it so hard to see that the upper level rubble location is similarly not part of the building rules?