Always welcome. Have a safe trip 😀

Emily, carry your canceled passport with you and try to use the visa in it at Immigration, if they reject it then just ask for a visa exemt. When you land you will need to fill out an arrival card. Fill out 2, one with the visa number and the other one without. If your visa is rejected just hand over the other card to the Immigration Officer and ask for a visa-exemt entry. They can just cancel out the visa number but they might also ask you to go and fill another arrival card in then you lose your place in the queue... The second card pre-empts this.

Yes. You should have no problem going to KL and coming back the same day. Carry the money like I mentioned above to show in case you get flagged for a check. The last time you got stopped was probably because of overstay. Also while you should not have any problem it is still upto the Immigration Officer to grant/refuse you entry. Denial of entry is very rare. Wish you luck!

If you have not been blacklisted by Thailand immigration for your previous overstay then yes, You can reenter Thailand on the same day. Since this will show as an IN/OUT bounce you may get flagged for a check. Please MAKE SURE you are carrying 20,000 baht or equivalent foreign currency with you just in case you are asked.

8-Major-new-crackdown-on-visa-overstay-in-Thailand-starts-today

Doesn't work like that! If you are caught on overstay you will get arrested and detained. In all probability you will end up missing your flight while your case is processed. You will be fined and incarcerated till you pay and get a new ticket. And as you cannot do all that in custody, if nobody helps you you're pretty much in deep shit. However please feel free to test the system yourself if you doubt all this.

Not recommended! If you are stopped by the Police within the Kingdom and you are on overstay you will get detained.

Please cut the corner of the front cover to void the canceled passport.

Both of the above can be extended for 30 days at your local Immigration Office.

A Single-Entry Tourist Visa is obtained from a Thai Consulate abroad and gives you 60 days when you enter the kingdom.

I think what you are talking about is a 'Visa Exempt' which you get on entry to the kingdom, which is for 30 days.