Insulin, by itself, is not an effective anabolic. (If it were, diabetics would be huge.) Insulin, in combination with anabolic steroids and resistance exercise, may trigger maturation of satellite cells (small, more or less useless cells that are held in reserve, which do not contribute to muscular strength) into mature muscle cells. The advanced steroid user may have plateaued in that respect, and adding insulin can give new gains.
I don’t think anyone knows whether short or long acting is more effective for building muscle. They seem to be about equally effective. A problem with long acting appears if it would continue acting while asleep: one might die if insufficient carbohydrates had been taken.