It may help your case, however you need to have an attorney help speak to the prosecutor and arrange for a substantial agreement offer. Also, you need some protection against being prosecuted for this information if the information would incriminate you in any way.
Typically, a mitigated sentence means that the sentence would be lower than what was initially imposed. The judge may give him less of a prison sentence which would mitigate the prison sentence, but give him probation, something he did not initially have to complete. Mitigation usually needs to happen within sixty days from the time of the judgment.
Yes, your records can be sealed since adjudication was withheld. After your records have been sealed for ten years, then you can expunge your record. Government can always see that you have had something sealed or expunged so you should always be open and honest re the charge otherwise that may prevent you from getting a government job. The private sector cannot see the charges at all.
You were not convicted, however, it sounds like you never had this matter sealed from your record. You would need to file an application with the Florida Department of Law Enforcement seeking the case to be sealed, then file a Petition and Affidavit with the court to seal all the records. Note that this can only be done if you have never been convicted of a crime.