International and Domestic same terminal. I've been flying domestically for 18 months. Always stacks of taxis available. I rent cars from Suvarnabhumi about every 3-4 weeks. All counters have been open throughout covid. Only difference from "normal" is the prices have been so low. Rent car for $
Nothing to do with Elite Visa, which is essentially a retirement visa for the under 50's. This is a working visa, but the financial criteria will price most "digital nomads" out of the equation
Possibly, as your extension would only be given up to the expiry of the insurance. However some immigration offices can be pretty anal when it comes to extensions. Suggest you enquire at your immigration office
You need an ONWARD ticket, NOT a return. Get a Flexi ticket to wherever you plan to go next within the timeframe of your entry stamp and change the date when your dates are firm.
With my lady, she provided statement of service from her employer, details of salary, length of service, bank statements, deeds of ownership of house and land, and that did the trick. She also had two previous visas to Australia which I sponsored going back about 4-5 years, so she included those details too. It was a breeze, no problem at all.