Interviews are popular in Asia and not so popular in Europe/USA. Basically a brief interview to confirm what you have submitted. Some embassies it is compulsory regardless of DTV option you choose.
Oh you have to show employment via a registered company. You should do soft power then. Provide 6 months course information and do not apply at embassies like Taipei which request employer registration for SP applicants
Working for your friend is fine, you need to submit contract of employment, salary evidence etc. you can show your stock account as additional evidence of financial stability if you wanted.You don't need to use an agent for SP.
The 500K THB needs to be in an immediately accessible personal account. If you are in ND then applying there yourself is best. Apply with your online teaching job. There should be posts in the threads from ND applicants