Unfortunately, this is another testiment to the truth of don't do too much too fast. I was afraid that adding that much Borax was gonna overshoot your target. It is always best to go slow with additions of anything. Several small additons that gradually get to a target are most always better than one huge addition. Fortunately, pH of 8.2 is not a dangerous thing and can be easily remedied with the use of some muriatic acid. I would suggest adding some baking soda first however to raise your alk up some before adding any acid.

I've never seen Borax damage a liner, so not sure what to say about that or even that it is actually is from the Borax.