Once the tightener has done it's job the 19mm bolt that locks the tensioner in its place will hold the tension.

Bad luck. quite a job to replace it. Did the same thing myself

You will be able to use the car until next time you want to retension the belt or change it.

You actually need to loosen the 19mm bolt to operate the tightener. If you don't...... damage to the tightener.