If I go to Chang Watana (government complex) for visa renewal and 90-day reporting, how late in the day can I get there and still get queue numbers? Will they process through everybody who they give queue numbers to in the same day, or do they ever make you come back the next day?

Not sure if there is a clean yes/no answer to your question.

Friend of mine did renewal of his Extension of stay and a new mutiple entry stamp in mid March this year.

He started one day at about 10 am finished the next day in the late afternoon.

I wouldn't risk much later than 13:30 - 14:00

It all depends how many people are ahead of you. If they think there is time they'll give you a ticket and process you that afternoon. If they think the queue is already as much as they can handle before closing time you'll have to go back another day.

http://bangkok.immigration.go.th/images/QueueCard.jpg