it's worth a try now with the new system. I'm on an Elite visa also and have never been able to successfully do 90 day reports online over the last two years. Recently (2 weeks ago) I registered in the new system and was able to complete the 90 day report online. I received an email confirmation after 6 hours that it had been completed.
I caught a bus from Vientiane bus station to Udon Thani bus station on a Tuesday around 4 months ago. Whole thing took around 2 hours and 15 minutes. Was hassle free.
Additionally make sure they know it is an ED extension cancellation when you first get a ticket. Initially I was given a form to fill out and told to go to the wrong area. Approached another person as I felt it wasn't right and was given just a ticket to section N2. All up took around 1.5 hours on the day I went.