I have another broad question for the experienced Evocators… Have you found that giving the spirits maximum choice and leeway in how to accomplish a task is the most effective approach for reaching a certain goal?
There is a legal issue that I am trying to help a friend with and I don’t honestly know all the different things that could possibly happen to swing the case in his favor. Is the best approach in a deal like this to just call the spirit and say “this is the end result we want, do whatever it takes but don’t cause any harm or misfortune to him or the people he cares about.” ?
Yes, this is the best approach. As a rule of thumb, the fewer restrictions you provide, the faster and better the outcome.
You could also, you know, ask an attorney to help. In what state is your legal issue and what field of law does your friend need help with?
He has a paid attorney. He apparently had an “improper relationship” (lol) with a 16-17 year old girl. It wouldn’t surprise me if he did nail her, but having a record will make life unreasonably hard for him. In TN by the way.
I see from another posting that you are an attorney. Do you use magic in your work?
Of course I do. 
I know, I know – way to perpetuate the “evil devil lawyer” stereotype… On the other hand, after having dealt with some human lawyers and corporations, a demonic pact or two actually seem rather benign by comparison…
In regard to your other question, try evoking Paimon and/or Belial with the goal of winning the case and/or making the case disappear. Should work.