Can a furnace high limit switch be bypassed for testing, and is that dangerous?
Yes, you can bypass the high limit switch for testing. It's as trivial to do as it sounds.
It's the acme of foolishness to run it like that for any length of time, though, and I think you already know that it could cause more damage. The high limit switch is there to protect against the type of heat damage you describe, along with fire. I'm amazed that metal melted without burning down your house. And yes, CO can build up due to a clogged heat exchanger, which is probably what tripped the limit switch to begin with.
I'd get a different repairman and suggest to this one that he's lucky you aren't suing him for criminal negligence or unintentional homicide. Chances are what he did was actually illegal.