Thanks, you're very kind about the drawing.
All your recommendations make good sense, and isolating at max train length in either direction from crossover B should work well, as I'm not looking to run anything over 90" max. Leaving the crossover A and the upper portions of B at matching polarity with the loops they connect to will simplify the set-up and lessen the need to worry about two trains conflicting an auto-reverser. Excellent.
Yes, I'm planning to use plenty of drops, and single controls for each of the crossovers, but am glad to hear that advice reinforced.
I will give some thought to the multi-operator scenario, though at the moment it's just me and mine. As for isolating the section under the overpass: I was under the impression that a DCC engine operating in a district that experiences a polarity reversal will continue to follow directional cab commands independent of polarity--what then is the advantage of isolating that area?
I may produce an updated drawing based on your (and Ed's) guidance and put it up to have you confirm I've understood properly, and if so, it may be of future use to anyone else crazy enough to try a layout like mine.
Again, thank you for the explanations & input. VERY helpful.