1/ They do not care where the funds come from, as you can apply using a sponsor if you want to. 2/ Yes 3/ Not only would it help, but it is essential that you show income to prove you can support yourself. Just showing invoices is not enough. 4/ Enclose your latest Spanish tax return, business registration, a letter on company letterhead permitting you to work remotely while in Thailand, and a covering letter stating that you will stay in VN until the visa is processed, you will not work for any Thai companies or citizens, plus you will abide by all Thai laws while in Thailand. Use a free website like ilovepdf to merge all your documents into one pdf for uploading, as the application site does not support multiple document uploads. Buena suerte