ALL:
If I have any question regarding the Zin(fRX - fLO) characteristics, it is more on how we can take advantage of it. If that function is a steep concave, particularly at lower fLO like 3 to 10MHz range, the BPF bank can be made with a single inductor optimized for higher bands and let the mixer reject the unwanted frequency. That would be very nice. It sounds a bit too good to be true to us but it would also be a big strike against someone trying to build a high sample rate wideband receiver.
Ryuji Suzuki AB1WX