When it comes to Instagram once the user have email control they have full control of the account. I will gladly allow an trusted user to change the password on my email until repayment.
Your collateral is worthless. Instagram does recover accounts especially with such a change of hands of accounts and weird activity (eg change of ip, quick to change password, etc). All it takes is a simple contact to them and they will recover it.
Offer real collateral that is 120% of what youre asking or lock this thread.