Your case never gets dismissed because you were not given a Miranda warning. However, evidence against you may be excluded because you did not receive this reading of your rights before giving incriminating statements to the police during custodial interrogation which may lead to a dismissal of your case.
No, you are not required to take a sobriety test when stopped, but Virginia law requires that you have a sample of your breath or blood analyzed once arrested and if you do not do this, you may be charged with Refusal.
Yes, you do.
There is a huge difference between a misdemeanor conviction and a felony conviction although both will stay on your criminal record permanently. If convicted of a felony, you will lose your right to possess firearms, right to vote, your right to serve on a jury, plus the loss of other civil rights. Furthermore, as a convicted felon, you may have great difficulty obtaining employment, especially if attempting to obtain a professional license.
An expungement is the removal of an arrest incident from your record where there were no subsequent convictions. It is accomplished by petitioning the Circuit Court. You may not expunge a charge if you were convicted and found guilty.