Hi,

Is 1 handed combat viable ? or does it get you killed really fast ? Lets say 1 sword in a hand and nothing in the other .

It can get challenging at low levels with your empty left hand, but...can i know why? you can have a shield, a spell, or another weapon; it can get pretty powerful, but there always better options than what you described.

I have a blind swordsman build who plays like this...an empty left hand means you can block with a 1 hand sword which is useful to break opponents guard

The highest DPS weapon-type is the dagger, so yes, totally viable.

It is viable - not using anything in your left hand allows you to block with a one-handed weapon. Blocking with a sword isn't that much worse than blocking with a shield, and I believe that most perks in the Block tree still apply.

I mostly play with a one-handed sword and nothing in the other hand. It's perfectly viable.

I find shields and two-handed swords make my character feel a bit clunky and less agile... even though those playstyles may be a bit more practical. I really don't like the idea of dual-wielding swords like a character in some cheesy hollywood action movie. And magic in one hand is a no-go, because I don't like roleplaying magical characters.

How does this make any sense?
Think you misunderstood what he's asking. He's literally asking about using only one weapon; no shield, no dual-wielding, no spells. Just one weapon.

I play the same way, my character is a ranger so his main damage comes from bows but I use a single 1 handed sword a lot as well and it's still deadly. I also use some mods which make combat much harder and it still works well.