Should I plead guilty to a misdemeanor hit and run with injury charge if they refused an ambulance at the scene and didn't go to the hospital until 2 days later?
For any attorney to tell you whether or not you should plead guilty, they will really need more facts. When deciding whether or not to go to trial, you should weigh the evidence against the risks of trial. A plea bargain is a guarantee. Even if you prove the people weren't injured, you left the scene-- when they could have been injured. Many juries will not have much sympathy for this kind of conduct. However, if they come across as con-artists, then a jury may reject the charges in their intirety.
If you don't have any attorney, you really need to at least do a consultation with one or two. Take them the offer. Take them any discovery packets or offense reports you have a copy of. From there-- they can review the whole situation and let you know if the risk of trial is a substantially greater risk. They can also give you an idea of what is a routine sentence in your jurisdiction. Doing some extra information gathering will help you make a more educated decision.