A QBI deduction carryforward is a QBI "loss" carryforward. It is sort of the opposite of a deduction.
No, nothing happens when you end the business. It keeps carrying forward until it is 'used up' against other QBI profits.
For your question about your rental property, opinions vary if a single rental is a QBI activity or not. My opinion is that a single rental usually is NOT a QBI activity, in which case it would not have any affect on your negative QBI carryover.
