Can I overstay my tourist visa in Thailand if my embassy doesn't provide an extension letter?

Wait for an announcement tomorrow. Or go into overstay mode, which MANY will. If you are at the south, the Malaysian Consulate at Songkhla will give you an extension letter. If you are at the east, central or north, tough luck, the Malaysian Embassy wont issue anymore letters. Another option is travel to Chiang Mai immigration, they will extend your stay without an embassy letter IF you have not extended before from the time you last came in until amnesty.

You are Malaysian, so you can just go to the Southern Thai border by train, or bus, and they will let you cross the border back into Malaysia. You just have to show your Malaysian passport.

It's 500 baht a day fine up to 20,000 baht. That's if you declare yourself. If you're caught by immigration police, even just a week overstay, it can be arrest and five year ban. Your choice

Ffs, just go home already.