I keep looking for some quick way to return playback speed to 100%. I finally got around to beginning the videos, and I just saw the video guy click on the box and enter 100 manually. Well, I guess there isn't a trick waiting to be discovered. There should be.
If I were coding this, I'd go in with a plan to have a variable playback speed like it is now, with some clickable buttons to set pre-selected speeds, such as 25%, 50%, 100% (probably user-editable, quick select A, B and C with these default values), with logic rigged so that only one button can be checked at a time, and no button is required to be checked (which rules out QPushButtonGroup it looks like). If 100% is checked, playback runs at that speed, and the variable control is disabled, but its value remains unchanged. Un-check the 100% button, and now the variable control is enabled, and playback goes to whatever the running speed was.