"Out of sheer luck, I measured VCC with the 'scope and found it to be oscillating at 16MHz! This probably caused my ALE and /PSEN signals to appear as oddly as they did.

My question: What should I do to isolate my circuit so that my ceramic resonator doesn't corrupt everything?"

It is not your ceramic resonators, it is your power.
The way the '51 is working all applications require a 10-100 nF low inductance capacitor between Vcc and Ground LOCATED AS CLOSE AS POSSIBLE TO THOSE PINS and most requre a 4.7- 22 uF tantalum in parrallel with the capacitor. However even without these capacitors your Vcc should be stiff enough to stay above 4.75 volts with the processor running.

I suspect you have a problem with (the design of) your power supply.

Have fun,

Erik