Security Deposit - and Furnished Apt

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Let me lay out facts. Someone please reply - I would be very grateful.

I rent a co-op apt. from an individual.

Paid security deposit of $2500 in June 2000
Rented the apartment furnished - with lots of odds and ends but there is NO mention of any individual item - or the fact that the apartment was furnished in lease or any other document

I had verbal agreement to leave apt. one month earlier and move across hall 8 months ago. Landlord reneged and said that he would keep security deposit (he said it was only $1250 (half months rent) - which I double checked later and the figure was actually $2500 - full months rent)
for the last month thinking he was doing me a favor by allowing me to leave early but only keeping half month rent - it is in fact however full month - the $2500 (for which I have cancelled check)
He has not kept this in joint interest bearing account (the building has more than 6 units - but he only rents this one out as he only owns this one)
He wants to come into the apt this Saturday - and I verbally agreed (nothing in writing that I would move out and leave key) but now that I realize that he in fact owes me $1250 I don't want to give him the apt.
Questions:

1 - Does he have to keep the security in an interest bearing account if he only owns and rents one unit in a 200 unit co-op building
2 - Do I have to move out per my verbal yes - if I called back and said maybe not - and there is no written release either. He may have taped the original call? He might show up with movers this Saturday (May 1) - but I have lease that says I actually have till May 31
3 - Can I change the locks?
4 - If I stay in apt. and show police lease and he tries to force me out - will the tape of a potential phone call trump the lease?
5 - As there is nothing in writing that the apt. was furnished can I take some of his stuff "for ransom" until he gives me back the $1250 that was promised? Can I be sued - can't I just deny the presence of specific items that he reports missing? There was NOTHING that says any items existed or that the apt. was furnished.
6 - If I don't let him in this month and he keeps the security deposit and I stay but do not pay him for this last month - he will still owe mw $250 (the monthly rent was lowered to $2250 in the year 2002) plus the interest on an account he never actually kept records for? How can I get this from him - if he was supposed to keep the account in a non-co mingled interest bearing account and he did not - what are the remedies?

Thank you very much. I am happy to talk on the phone about this if someone has good answers.

Thanks again in advance.