FRom12GO only a reference point. > Its best to go to the bus station the morning you are leaving with a copy of your passport as they record your details. Buy the ticket at the bus station and arrive early for the departure time . The 1630 above is shown on 12Go website however the bus may leave upto an hour after that. Happy travels
I don't have a timetable however you will have time to go to the bus station during your down time waiting for your visa. Unless they have changed you can only buy the ticket on the day of travel. Another hint is be early for the bus as its free seating and its a mad rush to board and retain a seat. They do pack passengers in like sardines to get to the border and you could be standing if you arrive at the bus station ontime for the bus
The international bus will get you to the bus station in Udon Thani then you will need to get a local transfer to the airport you can get evening flights upto 9 pm.