Update: <<Just yesterday I happened to notice a vacuum line half hanging off at the evaporator cannister below the driver-side headlamp. (also an emissions component BTW) So it's WAY too soon to know whether this will address both the rough/inconsistent idle and cold-engine knock under load issues.>>
This did not address the idle issues however a hissing sound (only audible when hot) has developed and this only confirms the presence of a vacuum leak.
The Haynes manual pointed to EGR and/or vacuum leak as probable causes for both the ping under load ("during acceleration") and idle ("loping") issues.
Thus far I haven't been able to easily locate the source of the leak. All visible portions of every line appear to be in good condition. I sprayed silicon lubricant around the EGR area and this may have caused a dead stall. However, repeating the test doesn't produce any consistent results. Definitely idles better when it's more humid or humidity is at 100% (raining,) regardless of temperature.