According to the USBC, any changes have to be even all around the ball, and they have to be done before bowling for score begins.
As for how much pressure, while at the lanes, it depends on how much you need/want to change the surface of that particular ball.
Yes, inhaling powdered resin can make you sick, but you're not going to create much by hand because the changes won't be that radical. If you really think you're going to make changes that night, I'd dampened the pad and put it in a ziploc type of plastic bag. But, you can't keep wet pads inyour bag all the time. Many people carry some Abralon or Scotch Brite pads in their bag on a regular basis. I carry SB pads.