My OM-D is on its way back. Looks like they did indeed find something wrong:
"Replaced top cover shoe case and shutter button unit, adjustment, firmware update, clean and check all."
For all I know when the EVF overheated it caused the intermittent problem with the shutter button. Who knows. At least now I know I wasn't imagining things.
One item that puzzles me with the above, the camera was already at the latest firmware so why the "update"? Or is that just standard practice/terminology when they do a repair?