I admit defeat. The Hot Glue wins.
I tried ironing, freezing and hot water.
I'm sure these methods work on some items. Just not on plastic buttons.
Ironing was messy. It left puddles of hot hot glue on the fabric and the buttons. I couldn't touch the button backs until the glue cooled a bit, which defeated the purpose of using the iron.
Freezing did harden the glue, which made it easy to pop the button off the fabric. Unfortunately, the frozen glue seemed to cling more fiercely to the buttons.
And the hot water? Didn't seem to do much at all. Each button is attached to the fabric with a large blob of hot glue. I don't think the water could penetrate enough to be effective.
Back to Goodwill they go.