The multiplier is a cool idea that maybe you could add, but might warrent explanation as it would have a huge impact on gameplay without being mechanically obvious. I think that spoils the simplicity more than lows would.
But on that note, lows could actually be really simple as you don't actually need to add a low block at all. Duck could double as a low block (as long as a low punch isn't being thrown by the blocker.) The ducking sprite already appears to be blocking face and body, so no double inputs required there. You could also make it so standing without using block will automatically elbow-block body shots, but that would be slightly less intuitive. Players would have to think less about specifically blocking a punch, and more about changing stances depending on which zone offensive and defensive play is most needed. Either case punishes block spammers and rewards the long ducking animation handsomely.
The problem then becomes "why stand at all?" Simple answer there too, head takes more damage than body, maybe with a multiplier for dodging. Maybe a hit forces them to stand. Whatever feels balanced and intuitive/simple.
Thanks for the game and the reply!