#1 - how do you know if it is loose skin or loose fat (simply more fat you have to lose?) see http://www.nowloss.com/how-to-get-rid-of-loose-skin.htm

#2 - low carb is a good strategy but all you are really doing is flushing out excess weight and eating less calories due to limited food options + a higher protein intake BUT... if you can sustain this for a long perod of time (which most people can't) then go for it.

#3 for weight maintaince (mass or weight loss) simply watch your calorie intake and avoid the fad diets.

#4 - as for gaining mass I suggest you look at tips 11-thru-19 here @ http://www.nowloss.com/fast-muscle-building-tips.htm but specifically you need to make 25-to-50% of your diet protein and you need carbs to "FILL OUT" your muscles and wthout carbs your muscles will look flat and not oned