1)"Multiscan" in this sense means that it can do 15-xx kHz and 50-xx Hz, something that isn't trues for practilly all new monitors.
2) Will do all old games, unsuitable for modern apps.
3) A scandoubler will only make sure all modes can be displayed on a VGA-monitor without changing the quality of the signal
A FF will do the same and remove the flicker from interlaced modes.