As I read through this, it seems to me that all of these points are good. Fundamentally, I suspect that there is not that much for the BB to do, no matter where it is. In a mature tank it will rapidly expand to the level of the food supplied (like snails), namely: ammonia. If ammonia was a big presence, I suspect that we would see quite a lot more deaths with even slight disruptions to substrate and/or filter media. We would also all be monitoring total ammonia with great frequency and concern. I don't remember the last time I checked TAN on my display tank.
Anecdotal experiences: I have always had overly-stocked aquariums, with and without plants, and cannot recall a tank wipe-out that could be attributed to ammonia ONCE the BB was established. Before we knew about the Nitrogen Cycle, we threw our spent charcoal and floss out every week and completely cleaned our filters, with no deaths. This was well before CO2 injection, so pH was generally ideally suited to leaving the ammonia in the deadly NH3 form, as opposed to the safe NH4 when pH is below 7. In this case the BB was probably never established in the filter. Often, I'd go a month or more before cleaning, which allowed BB to build in the filter - then threw it all out, with no problem. I have also completely replaced substrates with no bio-media in the filter and, again, no deaths. I've even been without power for days with no problem. In this case the BB in the substrate, and the plants, were undoubtedly critical. This might even make the case that plants take most of the ammonia.
In other words, in a mature tank, I think that BB rapidly meet the ammonia needs of the aquarium on any surface they can find (maybe even in layers) and, if you have plants, that further reduces the ammonia threat.