Would you pay for a warranty heat exchanger swap when nobody can actually find a crack?

I would not green-light an exchanger swap with no defect found. That's paying to fix the part they couldn't prove is broken, and if the CO is coming from somewhere else you're out the money and still have the problem. Nine times out of ten when a furnace this age shows creeping CO but the exchanger looks clean on camera, the cause is something simpler. Before I'd sign anything, I'd want three things tested. First, gas pressure at the burner. If it's firing hotter than the nameplate calls for, CO climbs and the exchanger gets stressed, and adjusting it is cheap. Second, temperture rise across the furnace compared to the rating plate. If the rise is too high, the furnace is starving for airflow, which means a dirty filter, a weak blower, ducts that are too small, or too many supply registers closed off. Third, draft through the vent and a look at where the intake and exhaust pipes terminate outside. If those pipes are too close togather the furnace can breathe back its own exhaust, and that alone will wreck the readings and shorten the unit's life. There are also two quick ways to check for a real crack that don't involve a camera. Measure CO in the air coming out of your registers while the furnace runs. If the flue is bad, it shows up there. Also watch the flames through the viewing port when the blower kicks on. If they waver or roll out of shape, that's a leaking exchanger. Clean, steady flames and clean air at the registers means the exchanger probably isn't your problem. On price, swapping an exchanger is a full teardown of the furnace, most of a day's work, so a four-figure labor bill isn't outrageous if the part is actually bad. But doing it blind is a gamble. Get an independent tech out first, ask for the combustion numbers before and after any fix, and have them written on the invoice. If a pressure adjustment or venting correction brings the reading down, you just saved yourself alot of money.

If you do keep running it in the meantime, make sure you've got working CO alarms on each floor.

I'd push back on the flame-watching trick. On my old furnace you could barely see the flames through that little port even with a mirror, and when the blower kicked on they flickered no matter what because of how the burner box was shaped. That test told me nothing and I burned an afternoon staring at it. A tech with a combustion analyzer will give you an actual reading in ten minutes, which beats guessing at shadows. Also worth asking your original company if they logged a CO number at past tune-ups. If you've got last fall's figure to compare against, you can see wether it's creeping or just drifted up since the last cleaning.

One thing I'd want pinned down before anything gets ordered: the actual number. 'Creeping up higher than he liked' is not a measurement, and some techs get jumpy over a reading that's still well within normal range for that furnace. Ask him to write down the ppm, the O2 percent, and the flue temperature after a full ten-minute burn, not just at cold start, because a cold furnace always looks dirty on combustion. If the readings are only mildly off, that paperwork becomes your baseline for comparison after whatever fix gets done. I'd also ask whether this is a condensing unit, since the secondary exchanger on those is basicly invisible to a camera and that changes what an inspection can honestly rule out.

Update: took the advice about getting the actual readings instead of trusting a vague warning. Second-opinion guy measured it and said the number was elevated but not alarming, then tweaked the air shutter and it dropped a lot. Still creeps back up when the blower kicks on though, which makes me wonder about airflow restriction. Would a clogged filter or blocked returns do that, or is that a stretch?