I, personally, think that MB is ok for men to do. It is a good, healthy way to rid yourself of pent up fluids, as well as stress, tension, etc. from time to time. That's the trick, though, is keeping MB to a occasional basis. Once it becomes a daily habit or obsessive activity, it's probably time to cut back a bit. It shouldn't be a major priority in life.
Precum is a fluid that is discharged from your penis during arousal, whether you are MB or not. Some men get very little precum, while others leak it consistently. It is a clear, semi-sticky fluid (consistency of lube, really) that is nowhere near as sticky as cum (at least, I assume, for most men). Meanwhile, cum is the mixture of sperm and semen in a sticky, white fluid that is emitted at the point of a man's orgasm/ejaculation. For some men, it is shot out of the penis at ejaculation, while other men tend to cum with a dribbling effect. As long as it coincides with orgasm, it's most likely cum (though small amounts of cum may be released out of the penis due to prolonged arousal, say, if you are edging for hours on end and reach a point too close to climax).
As far as the level of erection following release/orgasm caused by MB, it varies in men. Some men may stay hard for a few minutes or longer with continued arousal; however, chances are, if a man is no longer stimulated after cumming, his penis will go soft relatively quickly.
Hope this helps.