I thought that Slow Optimize is supposed to prevent the screensaver from activating.
It depends on your PreventScreenSaving
setting. Your second question indicates that the setting is "no", and in that case I do not know why your screensaver started.
2. Slow Optimize seems to change screensaver activation time to 1 minute. When I exit Slow Optimize, screensaver time is not set back to my original setting.
Yes. This is exactly how MyDefrag is blocking the screensaver, it set's the timout to zero. Windows will display this as 1 minute, but it's actually zero and the screensaver will not start.