I am in a negative equity position (that is, I owe more on my mortgage than my property is worth). In the event of a successful acquisition by ClozeSure, am I still liable to lenders for any shortfal

Yes, the transfer of your property to ClozeSure must be free of any mortgages.

 If you are in a negative equity situation, you will need to negotiate and agree repayment terms with your lender(s), because your mortgage(s) on your property must be paid off in full as a condition of your sale (both to your original buyer and – if your buyer defaults – to ClozeSure).