A Scan doubler allows you to display Amiga 15Hz screen modes on a standard computer monitor - it effectively doubles the scan rate to something that all monitors can handle.
A flicker fixer gets rid of the flicker that you get with interlaced modes - these will still flicker if you use a scan doubler without a flicker fixer, so you are right, you will miss rock-steady interlaced displays.
I don't know about the digital bit capability of them though - sorry.
