When did you try to book? I did mine a couple of weeks ago, booked online a couple of weeks before that. Not sure how you think you van book that far in advance, normally they let you book online about 1 month before, no way could I ever book 100 or 145 days in advance. (You used to have to sit up until midnight 30 days before you wanted the queue appointment as they were usually fully booked in a few minutes 30 days ahead. This year had been the easiest to get an appointment, could pick almost any day within the 30 days, and about 4 appointments per day - I thought the site wasn't working as normally it is full)
Something has definitely changed, you are correct, the marriage extension has disappeared from their site. I'd give it a week and see if it changes back again, they probably arsed something up on the site and might put it right once they wake up )don't hold your breath)
The Lamphun site is still offering all of the marriage appointments, got to be something to do with the incompetence at CM - These people amaze me, they spend years in a total mess, they eventually get a system that works then go and totally F@#$ it up again!
I checked the Lamphun site and it is the same as CM used to be, you can only select the current month or next month, there is no option to book anything further than April.