I was just rereading the comments having spent enough time doing research that the lingo doesn't seem so foreign!
I'm not sure why I didn't pick up on this initially but when you said that "nothing in the service tags make it seem like it's operating efficiently", does that include the most recent tag from after the furnace incident?
It could well be my recently elevated paranoia about CO, but I'm still finding myself struggling to catch my breath.
Also RE Overfire Draft, none of the evaluation tags since 2009 include a reading.
Thank you also for your input. I had no understanding of the workings of a heat system prior to a month ago when all of this unfolded so I had no idea what I should be in the lookout for.
Those days are over. Moving forward, which I feel blessed to be in light of recent revelations, I will be certain to enquire about and monitor the heating systems of anywhere I live.