Each state bar is different.
My husband joined the Indiana bar and he was required to submit along with his bar application a fingerprint card (for an NCIC background check) as well as driving records, and copies of any arrest records (he had a slight run in with the law when he was 18 years old).
The state bars generally don't have the time to check out every reference on your bar application, but they will check out your law school application. If you purposely leave something off of one or both, chances are, you'll get caught. Indiana also has character and fitness personal interviews that require the applicant meet with a representative of the fitness committe personally and review all items on the bar application.
As for actual employment dates, they generally don't verify that. If it comes down to it, list only approximate years. That's what he did and it wasn't a problem.
Criminal/court records though, you don't have that luxury. YOU MUST go to the individual courts and get exact dates/dispositions/etc. if that's what they're asking for. Otherwise, it appears as though you're trying to 'hedge' the facts. If the records are no longer available, you still should list the incident and make mention that you attempted to get the information from the court, but there was no record. You should then also include a statement from the court indicating that they show no record.
Something else to remember - you're not going to be precluded from a law school just because of past indiscretions, unless they are so absolutely horrible, that the school has no choice. The purpose of them asking those questions is to make sure there is nothing in your past that will come out to haunt the school later. They need to be prepared up front.
Hope that helps.