The official government website is your local Thai embassy. Each embassy sets their own requirements, so anything posted anywhere that is not the place processing your visa application is irrelevant.
You don't have one. Leave it blank like people have been doing for the last 2 years.
And ignore anything you read on the website you linked. It's from an agency that's just trying to drum up business for themselves and CONSTANTLY posts incorrect or outdated information.
There's no such thing as a "DTV card". It's not a membership program. It's a visa. And the pdf you received from the embassy is your visa. You need to print it and keep it with you for 5 years, and show it each time you enter Thailand.
It does not require details on your outbound flight. That section is not required. Don't touch any of the questions about the outbound flight and it will remain not required and you can leave it blank. Start filling it in and it will become required.
Not required to show international transfer for marriage visa or extension. But it needs to be in your bank for 2 full months before applying for an extension.
You can use the money as soon as you finish the "under consideration" period and get the full extension stamp in your passport.