I submitted all 3 for my application but different embassy. I guess you could just do 1 and 3. Important to show work payments received along with the 500k.
You can apply in one country for the DTV and enter Thailand with your kids. To obtain add on DTV for your kids you must apply outside of Thailand. Very likely your money is enough as your kids are very young.
It will be better if you "borrow" the 500K from your mum, deposit it in your account and apply yourself. Embassies do now ask about income for SPers but there is no minimum income requirement,
Yeah it is not necessary to have an agent. Sponsorship via the main DTV holder is easy in Vietnam and other countries. Sponsoring an adult is much more difficult. Some embassies will only approve adult sponsorship via an employer not family or friends.
Jakarta has approved a family with just the main sponsor s 500K. Your children are very young 3 and 2.. Your application is strengthened if you can show more than 500K.
There is no appeal process, You can reapply again as others have done. Applying for a student visa will not be affected. Seems your paperwork has a major issue going by the length of time to reply.
I got the DTV with a week apthotel booking. Malaysia requires a minimum of 6 months accommodation booked. Vast majority of embassies have no minimum requirement.