I don't think that getting used to the test format and timing is truly "prepping". When middle kid took the SAT in 7th grade, she went through some practice problems and did one practice 25 minute CR section and one practice 25 minute math section (refused to look at writing). Eldest didn't want to take the ACT but I had signed her up. She did one practice test the week prior to the actual ACT - I think that is reasonable so that you know the test format and the timing of each section.
As for accommodations, I have mixed feelings. Most kids would do better given a bit more time. I have an issue with kids who are scoring 2000+ without extra time, then they get extra time and score 2300+. If you are already scoring 90th percentile or above, I don't see why you should get more time. However, I know some kids who truly need the accommodation (for valid physical reasons), and I have no issue with that.
