People being denied entry are not flying all the way to Asia. They are leaving Thailand to a neighboring country and coming right back, and trying to stay here long-term without a long-term visa.
As you saw, they did not stamp you in for 45 days as that's not legally possible. But their warning is clear. I wouldn't try flying in again for quite a while without a visa.
Every amphur has different rules and regulations for yellow book and pink ID card. At some it's a simple process, at others it's very complicated and takes lots of paperwork, witnesses, and many months. Some will refuse to do it flat out. You have no recourse except follow the process of your amphur.
2) you need to at least put flight information. Find a flight and put the information. Some embassies may require an actual booking
3) depends on the embassy
4) you cannot arrive on August 14th and apply for extension. You can only apply for extension when you have 30-45 days left on your entry stamp. You can apply for a single re-entry permit and then apply for the extension when you get back.