The problem is that you can’t just make antimatter. You have to make matter and antimatter in equal amounts, thus you don’t get to save half the energy by making just the antimatter and then destroying it with some spare matter you have lying around.
So, you are already just breaking even. Throw in some inefficiency in collecting the gamma rays from the annhilation process and you’re well on your way to obeying the laws of physics.
To answer your questions, then:
a) If you could do what you suggest, yes it would violate conservation of energy. It would also violate conservation of “particleness” or what is called Charge Conjugation. You can’t change matter into antimatter or antimatter into matter, and if you create or destroy stuff it has to be equal parts of each.[sup]*[/sup]
[sup]*[/sup]So, how did we end up with so gosh darn much matter and no antimatter? This law (CP invariance) is broken in some very minor ways in certain esoteric particle reactions such as neutral kaon decay. Enough to result in a preponderance of matter in the universe, but I doubt you could take advantage of it for your purposes. First, the bias is very small. Second, you want antimatter and the bias is towards matter. Finally, particle physics is never able to create just certain particles - if you find a particle that decays into antimatter more often than it does matter, you’ll end up spending many many times more energy generating other particles and just plain heat and radioactive concrete trying to get them. So, you still are out of luck.