So you are well placed to just go in and out with no hassle. When i applied in person in Taipei, the Thai immigration officer who approved my DTV told me to just do border runs as it is so simple and no need for the paperwork.
No, never asked for that. If you are on soft power, it would be good to have proof of ongoing activity. Soft power applicants have been asked to show paperwork that they are still taking classes/treatment.
Yes, you need to include contracts of employment. Who gives you a contract? they can write a letter saying they are ok for you to work remotely? Portfolio incs resume, letters of recommendation/appreciation from clients. website info, sample of work etc.
provide outline of work, portfolio, letter from employer detailing permission to work remotely in Thailand. Highlight payments from work on bank statements Also check thsi site for lots of pointers