it is useful, for giving basic layouts and contents of a simple doc such a cert of employment. Many applicants use it to get their visa, as posts detail. AI also used by lawyers for creating drafts, contracts etc
They are out of their home country until next summer. Very large numbers of DTVERS successfully apply outside their country, without difficulty.The system is made for such applicants.
Singapore accepts applications from citizens and residents. If you are not one of those you can apply from Vietnam or Cambodia. Note Cambodia can take 3-4 weeks to get approved, Vietnam around 10-15 business days often quicker.
Very feasible, as long as you have the cash you are good to go. I know several people who do the same on DTV/other visas. Yes, it is safe to leave your belongings in your place while away. The ease of renting is one benefit of living in Thailand.