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First, in addition to replacing the 20a breaker, should I also switch the
15a
receptacle w/ a 20a? I only have the one receptacle on my circuit...

Acceptable

Second, what's involved in swapping my existing 120v circuit to 240v? The
compressor is currently wired for 120v but I do have the option of
converting it to 240v if I must. How much work will it take?

Not much, but it may require running 12/3 instead of the present 12/2


And third, how at fault is this electrician? Is this something he should
have
foreseen? How much of this is his responsibility? I don't like that he
passed off old as new. Or that he made excuses when he was found out.
But most of all I don't like the idea of giving him more money if he's
burning me,
deliberately or not.

As for responsibility, that depends. Did you tell him "I want to run this
compressor, please run the proper wiring" or did you tell him what to
install? A good electrician will know what is needed, will ask about hte
draw of the compressor, etc. He will know that when a compressor cycles, it
will draw far more than the rated amps for a second and install a quality
breaker to handle it.

As for using a used breaker, he save only a couple of dollars. Was money
the motive or did he just happen to have a used breaker and no new one? He
should have either told you or just went and got the right breaker. Unless
he worked very cheap, you got screwed. The fact that hte breaker is used
done not mean it is bad, but it does bring up the integrity of the
electrician. We of the use used equipment, but in an industrial setting, it
may meant thousands of dollars difference, not five bucks.