They have several years to file on you, so don't worry about that. Make sure you get an attorney to review any "deals" you strike with them as, most of the time, these deals are largely unenforceable. They'll contact you when they need you.
If he pleaded guilty to the theft already, they (the State) can't charge him with theft again. The Insurance company can't charge him with anything as they're just a private company, but they might be able to go after him civilly, for costs incurred in recovering that money. It seems like it would be a waste of time for them, though.
Appeals are extremely technical matters. Whether there is or isn't something that can be done requires a great deal of research, thought and expertise. I would strongly advise contacting an attorney to go forward with any more appeals.
Well you have a much better chance than not guilty. There are a lot of rules about the admissibility of the portable breath test devices. This definitely falls under the category of "hire an attorney to fight it."