I'm not sure that there is a valid grounds for an appeal in your case and a pro bono appeals attorney is hard to come by. I have provided a link to the Detroit Bar Association, try contacting them to see if they know of anyone who could help you.
An affidavit is a written statement of facts that is voluntarily given and sworn to under oath; it is used as evidence in a court proceeding. As to how much influence someone would have, it depends on what they are saying and if you can prove that they are not credible. Yes, an attorney would charge for this, but it varies.
Speak with your lawyer about your concerns with how he is handling your husband's case. If you still see no change, you may need to find another attorney. Contacting an attorney in San Bernardino County would give clarification on what is and isn't available at their local courthouse.
The age of consent in Michigan is 16, but there are some factors (like being in a position of authority over the other person) that increases that age. Also, you better be 100% sure that she is in fact 16 and not 15 and 364 days or you will have a big problem. It doesn't seem that the risk is worth the reward in this situation. If you must proceed, I would contact an attorney in your area (many criminal defense attorneys offer free consultations) and explain the specifics of your situation.
Keep in mind that true victims of domestic violence often recant their accusations out of fear for their safety- so many times these crimes are charged regardless of what the alleged victim says. Hiring a local criminal defense attorney will give you the best chance at a positive outcome for these charges.
Yes, people are charged with crimes they didn't commit quite often. Your friend needs to obtain a criminal attorney, who may be able to contact the police on his behalf and attempt to resolve things before he is charged.