According to Apple, the PowerBook G4 has a stereo line in and an integrated microphone (doesn't specify stereo or not). A balanced line mic plugged into that would give you the same thing on each channel, except it would be 180 degrees out of phase. A true stereo mic would give you stereo, but...
With that setup you are going to wind up with only 2 tracks on your computer which will basically be a stereo mix of everything that is fed into the mixer (keyboard, drum machine, mike).
You behaved professionally in every way. Some people are just assholes and you happened to be dealing with one. Best not to be involved with such people.
Throwing a public hissy fit before you have given the company to respond to the problem is a major poor move. Call customer service, explain the problem and see what happens. Everything I have gotten from Zzounds has arrived in perfect condition, usually the next day (it helps to live just 2...