The purpose of the final walkthrough is to allow both parties to be present to reach an agreement on what items might require cleaning or repairs and to give the tenant an opportunity to do the cleaning or repairs to avoid any deductions from the security deposit. The landlord may have waived the right to make deductions from the security deposit when he agreed there was none needed when he gave you the normal wear and tear rating. If landlord subsequently finds out that he left out some items which he may be entitled to deduct and the omission is not because of obstruction by tenant's personal belongings, he can't deduct the expenditures from the security deposit but you may owe him that money.
21 days is 21 days. That's the amount of time the law gives him. You just have to wait till the time has expired before you take any legal actions to recover your security deposit.
When you got a normal wear and tear rating from the property manager, that should stop the owner from changing her tune on the security deposit. If after 21 days you don't receive your full deposit, you should strongly consider a small claims action.