wrote

So, if this fix will see
me through this heating season, it will have paid for itself.

More power to ya. Then you can get 3 or 4 quotes from reputable companies in
the off-season so you aren't pressured as much. Do yourself a favor and
don't try to do it during the peak cooling days of the summer.

BTW,
what are the risks of running the furnace with a damaged heat
exchanger? From what I read, the CO is not that much of an issue since
the heat exchanger is under positive pressure from the blower and the
flue gasses just will not escape into the distribution air. Am I wrong
on this? I understand there is no real good way to check the heat
exchanger, but are there signs of a HUGE problem? Like if I look
inside and see orange flames or something like that. I don't, BTW, I
see nice blue flames that look as they should.

Keep a couple CO detectors up and "running" just in case......