1.- Is that the culture so different from western countries?

YES

2.- Is the heat bearable?

SOMETIMES BUT MAKE SURE YOU HAVE AIR-CON IN YOUR RESIDENCE

3.- Is it possible to live off 30000 bahts per month?

30,000 baht is SHYTE and while it's possible, you're not going to have any extras.

4. If I brought my girlfriend (something unlikely), would we be able to live with such as income?

Not unless she's working.

5.- The uni and the group I would be working with sounded excellent and really nice. Is Mahidol a, so to speak, prestigious university?

Yes, it's prestigious - hit em up for more money because 30K is really piss-poor pay. Who cares about prestige if you can't pay the bills?