If you are dealing with stress by masturbation, it's obviously not the right thing to do.
To deal with stress I'd suggest looking into Pranayam and Yoga practice. Daily practice of both will make you stress free.
This is completely untrue. There is simply no difference in the process (it's all friction) other than during sex it's just more pleasure and more of a release. Also during sex you have cardio pretty much, so it might be good for you just because of that. Sex is relaxing not because of the actual orgasm.
You don't get acne from the masturbation act itself, it's hormonal imbalances and fluctuations that cause it (that is if they even cause it, there are heaps of other possibilities).
When you are a teenager it's the stupidest thing not to masturbate at all, just don't do it every day and wash your face & hands/take shower afterwards. If you are teenager and have acne, look at your diet, sleeping pattern, actually interact with girls, but if it's still there, live with it, there is most probably nothing "wrong" with you.