If the charges are dismissed and sealed, they cannot be found through AMCAS's 3rd party criminal background check service.
However, during your clinical years, especially if your school has you rotate at a VA hospital, you will have a federal background check of sorts, which will reveal sealed/dismissed charges.
My advice is to answer the AMCAS and secondary questions about criminal charges and convictions truthfully, and offer nothing more until you're already matriculated. After you start school, you can tell them about it on the topic of your concern for rotating at VA hospitals and the background checks it requires.
Also, charges being dismissed is a decision by the judge, and the implications are that you were not found guilty. A crazy ex can call the cops and claim you assaulted her and press charges, but the judge can see that the charges are bogus and dismiss them. Same legal results for your assault charge.
For your plea bargain, you essentially pleaded guilty so you're SOL on that. Try to get all of the charges expunged. Speak with your lawyer about the technical legal outcomes (formal conviction?), although I'm fairly positive most lawyers will not have the time to sit down and go through each question with you.
Good luck.