You can subpoena the doctor without compensating him/her for the time, but you will not have a very cooperative witness. Usually, the doctor is not called but the records are subpoenaed and placed into evidence.
There is a big difference between subsection b and c. Subsection b is a violation, not a crime, and subsection c is a criminal misdemeanor. If you were given a ticket for subsection b, you can either pay the fine or go to the hearing and try to get it reduced. You really do not need a lwayer for this. The charge will not be changed to subsection c and nothing will appear on your record.
Probable cause is needed but may have been from the observations of erratic driving. You should be able to tell this from the criminal court complaint. Speak to the lawyer about getting a copy of the complaint. Sometimes, there is a traffic violation that was the basis for the stop but no ticket is given on the theory that the DUI is a more significant issue. This, however, is not a good practice from the prosecution's view because it does raise an issue about the validity of the stop.
If your lawyer cannot answer this question,you need a new lawyer. There are many signs, though they may not be definitive. The clearest sign is a codefendant who has pled guilty with a sentencing date adjourned until after your trial date. Or a co-defendant whose case is separated from yours without any clear explanation. Really, there are many signs and each case needs to be viewed on its own.