Let's see. From Brackett's script:

The man revealed is draped and hooded in cloth-of-gold, so that we cannot see his face, but the figure is instinct (sic) with power and Darth Vadar (sic) bows before it.

DARTH VADER
Your Imperial Majesty Highness.

The voice that speaks is silken and deadly as a strangler’s cord.

EMPEROR
You are aware of the disturbance in the Force?

DARTH VADER
I am.

EMPEROR
Then you know that Skywalker is not dead. And he is more dangerous than [unreadable] I had realized. Remove him this time, Lord Vader… Or I shall remove you.

Darth Vader bows. The screen goes blank. Vader turns away. From his posture and the rhythm of his breathing, he is in a rage because he is afraid of the Emperor.

CONTINUED

144 (cont.)

angry because of his fear of the emperor.

It seems like the Emperor knows about the Force, and this script was written 1978! I'm not sure if this was the original plan for ANH too, but it definitely seems like the Emperor was able to use the Force since TESB.

P.S.: there's a myth among fans that the Jedi weren't supposed to wear robes in the OT. According to TESB's screenplay...

The Emperor's dark robes and monk's hood are reminiscent of the cloak worn by Ben Kenobi.

...which seems to imply that Obi-wan's clothes aren't ordinary robes: possibly a vest, a jedi vest maybe? I'm not saying it was supposed to be the only "official" Jedi vest, though.