JZ, I totally understand your concern, but as far as I'm concerned, that is a non-issue until it is proven to be a problem. It is probably a minor factor in the mess, but it is not the dominant factor as far as I see. You can't get distracted by minor factors and lose sight of the dominant factors... that's probably why a lot of people believe RF is too much voodoo and little theory.
The BPFs are low Q because (1) they are designed that way, and (2) the interaction with the TX LPF's reactance curve makes the BPF even wider passband by partially undoing the series resonance. I thought I explained above. My posts were chronological and they might have taken people to a mini tour to my experimental journey, which I know can be mentally tiring a bit. I can rewrite more clearly after the whole job is done.
As you see in my traces with (1) my finger on the LPF, dampening/detuning the filter operation, and (2) shunt resistor, dampening the LPF impedance swing both show the BPF response close to theoretical one. If you saw a significant deviation there, you would next look into the next suspicious element down on the list, but I'm totally satisfied with those filter responses for a practical transceiver. I only wish we could easily decouple the LPF and BPF on this very tight board space. Without doing something about this LPF-BPF interaction, doing something about the detector impedance will not improve the BPF operation to any appreciable level.
Ryuji Suzuki AB1WX