Let me start off by saying that you're right that the agent should've let you transfer the stuff from the heavy bag to the lighter one. This is a place where you need to take control of your own destiny and just do it. At that point if they'd still wanted to charge you the fee, assuming you'd gotten the heavy bag below the limit, then you'd be justified in throwing a fit (and disputing the charge on your credit card).
In terms of seat assignments, it happens. It sucks. Often equipment swaps to a different type of plane or different configuration of the same plane will cause seats to be shifted. Generally these things can be resolved at the gate especially if you're polite and ask nicely instead of demanding.
Expecting a full refund for this seems more than a bit excessive. You could try to dispute the baggage fee and take your chances with the credit card company to see what happens.