Thai Vietjet Air. I can't book flight for same day on my PC, but I can on my mobile?

I want this flight because im near Suvarnabhumi airport.

Question for Tod/Robert and other knowledgeable members...

If I fly to KUL and return on next flight back to Thailand (same day) for border bounce on METV:

1. Does Thailand immigration require me to stamp into Malaysia before returning, as having entered another country or is it as simple as just having exited Thailand?

And:

2. Do you know if It is possible to arrive and return to Thailand by same day flights, in KUL, without clearing Malaysian immigration or customs - not sure if Malaysian immigration or the airline requires it?

Cheers...