One study I have heard of and not SEEN, said that glutes actually show the best absorbtion rates. It is probably the most common site to inject along with the deltoid.
Diet is a huge huge ball of wax, but I dont believe in very high carb diets myself. If you dont have your diet down, you are starting a bit premature I think.
How are protein shakes not worth your while? Count your protein along with the rest of your caloric intake as you always do.
Keeping your gains is also another ball of wax, training and diet go a long way, but water making up 70% of your muscle mass if you lose that water you are going to lose some size too.
I have gained strength while losing almost 1/4" on my arms and 14 pounds of "water/fat," it happens. I know an overweight guy with 19" arms like me, but I know its a lot more fat and water than muscle.