Well other interactions with immigration may be needed, like a residence certificate or passport change etc. they will check on those if you have filed.
You would only get a 30 day extension from a visa exempt entry.
Some offices will allow you to apply for a second time but you will be denied the extension and given 7 days to leave the country. This denial costs the same as your original extension.
You apply for the e-visa in the country that you are applying from. Malaysia has switched to the e-visa system so if you are there you can apply online.
If using an organised tour company that uses the PoiPet/Aranyaprathet crossing then you’re pretty certain to be Ok. They know the wheels to be greased.