it doesn't, it's as per the hotel confirmation, it shows check in and departure date. As stated above even if a flight is delayed they will go with the confirmation
yes very easy. If you arrive visa exempt they are currently stamping for 45 days, so after ASQ you have 29 days. Just visit your local immigration office and explain you want to convert to non O for retirement purposes. There is a very useful Facebook group call Thai Visa Advice. Join that and they will explain the process in great detail