1. Depends on the embassy you apply at, many have no seasoning requirement, others 3-6 months. 2. No visit required at embassy unless asked for an interview. Depending on where you stay in Vietnam will determine the embassy you apply from. Hanoi and HCMC cover different areas. 3. Key info will be showing you work online for non-Thai firm/clients, you work for a legit company, you have permission to work online in Thailand from employers.
That's ok to send for proof of business but the 500k THB must be in a personal bank statement not a business account. You do need to send a letter detailing you are the owner and can work remotely in Thailand.