As a TA, I used a Scantron machine a handful of times. A small tear on the side likely won't affect how your answer sheet is graded.
If you're absolutely paranoid, just have them manually grade it, as @"Cant Get Right" suggested.
I use scrap paper all the time. We do get that extra sheet in real test. I don't think there is any harm in using extra sheet of paper. If anything, i think this would just simulate test day condition. Go for it.
Yeah, these things are serious. My recommendation would be to practice PTs with the bubble sheet and have things completely finished by the 34 minute mark.
... stray marks on the bubble sheet. If you make stray marks ... stray marks on the answer sheet). Don't risk it, LSAC ... the number on the answer sheet.