Could I cash the security check under protest?

You do not indicate what state the property is in. In California, no interest need be paid on the security deposit; unless your state law requires interest be paid or the lease so states, you are not entitled to it. In California, the landlord must send the tenant within 21 days of their giving up possession the entire security deposit and an explanation as to why any sum is being deducted, but is still allowed to argue the right to a deduction if they do not comply with the 21 day requirement. You should send a certified letter to the landlord demanding a refund of the entire security deposit and why you are so entitled, wait a week or so, write on the check that it is a partial return of a disputed amount, and then deposit the check after making sure it will not bounce [warn the landlord that you will go after him if it does bounce]. After living their 20+ years, do you really think a deduction of $300 is unreasonable? In all that time you caused no damage beyond normal wear and tear?

No, you need to sue for the balance. Did the landlord give you an itemized bill? Generally the lease states if interest will be provided on security deposits. Generally their is a cleaning fee that are included in most lease for example if they will deduct for carpet cleaning and for any damages (repairs).