You have 14 days to submit the requested documents. I have heard it's possible to have an even longer time by request with viable reasons, but that was just one case at one embassy.
you can screenshot the final stage of the application data overview before submitting. You can edit all the fields, before submitting. I don't know if you can access a rejected or cancelled application to review its contents for clues to the error.
Anonymous participant 166 your extension application is also put under consideration and then you need to return to get your final stamp, and you must exit Thailand after 360 days regardless.
Check out Issa Compass, it's a software company that automates the application process, not a recommendation, just a suggestion, apparently they have a good success rate