You will need the CNI and it will need to be translated to Thai. The translation needs to be done by anybody who is recognised by the ministry of foreign affairs as certified translator
No a district office or Amphur will only issue documents in Thai, to get it translated contact the ministry of foreign affairs in your province. Make sure who ever translates it is recognised by the ministry and has a stamp of certification
I use google calendar on android at the top left is 3 little bars tap on it and it opens a drawer. Scroll down and you can check or uncheck holidays and you will see settings/ general in there is location. Sorry don't know what app you have
Go to calendar app in your phone in settings turn on or show holidays and make sure your location is set as Thailand. All public holidays in Thailand will then show