What is my tax position if my buyer fails to complete, forfeits their deposit and I receive the balance from ClozeSure?

Your tax position is unchanged.

Your tax position is unchanged.

By proceeding with ClozeSure, you will receive your original contract price (10% from your buyer and 90% from ClozeSure).  So, for tax purposes the position is no different than if you had received the full (100%) price from your buyer.