I understand that I can apply at CW for a 1-year extension of my (90 day) permission to stay up to 45 days in advance of the expiration of the 90-day permission to stay.

 

Is it correct to assume that I can also apply for a multiple-entry re-entry permit up to 45 days in advance of the expiration of the 90-day permission to stay? In that case, the MEREP would be valid for more than 1 year (1 year+45 days)?

 

The reason I’m asking is that if the above is true, I could plan a side trip from Thailand without having to wait for the old expiration date, I could avoid paying for a single REP and I wouldn’t have to return to Thailand before the old expiration date.

When you do your extension you have a new stamp. Permission of stay. 1 year from your current stamp. If you exit los obtain a reentry permit. Its date will be same as your new extension. Yes you can leave and reenter with that reentry permit prior to date on current stamp. BTW you can obtain your reentry permit at airport if it suits.

I believe you are correct. The extension will cancel the previous date of permission, or supersede it. A MREP can only be applied for once the extension is issued. 

When you apply for the extension you will get a stamp dated one year from the date the 90 day permit to stay you have now.

You can apply for a single or multiple re-entry permit as soon as the extension stamp isdone that will be valid until the date on the extension stamp.