There are a couple of rules of the road if I may suggest - this comes not only from flying but also working gate/ticket counter.

1) You have no idea who gave the seats away. The chances are the person you are talking to DID NOT give them away - someone at the counter or another agent.

2) It really does not do your cause any good to rant and rave. There is nothing you can say or do they haven't seen (I'll get you fired -- I am best buddy w/ Airline Pres -- I am etc etc)

So what do you do?

1) Show them the problem - in your case I would show them the BPs you had and say - well does this seem fair to you? If you can get them to agree with you, you are half way there.
Yes there will be a few duds out there that will lie about things but GA's, like most people, want to help -- especially w/ a Gold or better customer.

2) Waiting till the end is a good suggestion. What have you got to loose? You may get a little more slack if there are not 20 pairs of eyes watching what the agent is doing.

3) Keep the BP's originally issued with you. If all else fails you can then get on the plane sit in first and when the FA ask for your BP show them the ones with someone sitting in your seat.

Who knows? This is a different person and you may get to sit there.

4) If all else fails write a letter. And write to the VP of Markeing -- not the President or Customer Service.
Why the VP of Marketing? -- He's out there selling the airline to folks. He knows how improtant it is to keep the customer happy. Other folks who are in operations are real big on rules... and that's what the rule says - tough luck. or We had a XYZ problem and the answer is you two can't sit together.

A VP will probably be more reasonable.

Send a letter -- be nice in the letter and ask for a couple of upgrades to compensate.

Just an idea or two. Didn't mean to take this long.

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TANSTAAFL - but if you work it right, FF miles comes pretty close.