IME, GrabTaxi results in a metered taxi, for which the quote is an estimate. It can go higher based on traffic/routing. The meter is used. And you pay according to the meter.
GrabCar is a firm quote. I think JustGrab is a firm quote as well. (I've gotten a metered taxi using JustGrab, but the meter is not used.)
Pay at the end of the trip.
People take GrabCar from the airport(s) here. There is a slight risk, but I haven't seen any reports of confrontations at the airport recently. The embedded messaging app, with translation, seems to work for arranging a pick-up spot.
The "authorities" (Transport MInistry) seem on a track towards accommodating Grab. One political party (Bhumjai Thai) made inroads running on this, and other modern proposals, but dragging Thailand forward into the future is still a slog.