Well actually it would be more fair to have you be able to block just fine with one empty hand and a 1h sword. You can block a 2h weapon just fine with a 1h sword by turning your sword broadside and brace the sword with your empty hand. Of course it won't be as effective as using a shield but blocking with a 1h sword and an empty hand makes plenty of sense and would make it balanced up against fighting styles that have both hands equipped.
yes, and depending on the weapon your are blocking against, breaking your sword. A sword's strength is against forces from its tip, or its edged side if a slashing sword. Even a broadsword was not designed to withstand a broadside strike. That method would work against another sword or a simple staff, but not a mace, heavy war axe, or hammer. It puts your hand in danger, as you jar your wrist from the force of the strike if you arrange your hand in a way allowing the blade to not break.
It is better to parry with a sword than to block. Parrying, a form of redirective blocking, allows you to strike back quickly if executed properly. Blocking just stops all action for a moment, allowing the slightest chance of recovery before counter attack.