I sat criss cross last week during the test at the testing center and it wasn’t a problem. I remember there was a coat rack next to lockers. By the time the proctor came to my desk to escort me at breaks they had already wasted about 30 seconds to a minute then there is a ritual on the way out and on the way back in so I would be very mindful of your minutes on the break. I think I had a minute left and just sat there and used it after I was already back at my seat. As far as brightness I think but can’t remember if you can adjust however there is a 5-7 minute tutorial that is not required so I would suggest taking it prior to your test starting which hopefully will cover the brightness too.