Regarding skill level certification, my advice to anyone who asks is to put down the number that you think most accurately reflects your playing ability. If you just went up to a 5 but really think you don't have the ability of a 5, put 4. Remember, it's ability, not average. If you aren't sure, ask yourself "If I play my best, do I have a chance at that number?"
If you certify yourself at a number higher than what is in the database for that tournament (databases created a few weeks ago), you will be raised prior to the start of play. In most cases, you stay where you are on the pre-drawn bracket and just play your matches at the higher skill level. I have seen, however, a few cases where players were moved after certifying themselves up.
If you certify yourself at a number lower than what is in the database for that tournament, nothing will change but the certification will be on record in case a DQ situation arises.
Disqualification can happen if you aren't honest with yourself or with APA. If you play significantly above your level and APA thinks there's no way you didn't know you could do that, then DQ comes into the picture. Disqualification at a singles regional is rare, but can happen. Most of the cheaters don't play singles, since it's easy to draw attention to yourself in the singles program. There are a few, however, who think they can pull it off.