>> Having the BC junction forward biased when RB0 is driven low is an
inherent
>> part of the circuit design IMHO, and if this did not happen then the
>> circuit would never work at all, as then there is no way to pull the
>> R1/base junction down to a low voltage to allow detection of the other
>> switch's true state. The base-emitter junction does the same job as the
>> diode in series with the switch.
>
>Ding!  This is what I was expecting, which is why in the original (working)
>circuit, I removed the diode, and the circuit functioned just the same.

Yeah, think of the transistor as acting like a 2 diode OR gate, instead of
as a transistor. You could replace it with a pair of 1n4148 diodes, and it
should work in a very similar manner. However there may be an advantage in
using a PNP transistor in the manner that Olin suggests, I just have not
worked through that arrangement in my mental electronic gymnastics.

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