It was a great explanation, but for your purpose definately needs to be shortened!!

When dealing with models I've learned that being direct the best policy. In fact, the explanation of you next to last paragraph, explaining that it's your shoot! I would use your explanation in the letter -- it made you sound as if you mean business! I hate it when people waste my time by sugar coating everything! You are right about, "if they won't do it someone else will"....so instead of wasting time finding out that a model thinks they're in control -- leave no room for confusion, then they will know its your shoot!! That's more time you could spend taking pictures!