The interface is incredibly well designed if you know what you are doing and probably utterly inscrutable for modern players.
Apart from saving, you don’t need any of the icons on the left of the screen. To use a key to unlock a door (with no icons selected) left click on the key to use it, right click on the door. The door will unlock.
To use the door (or anything else in the world) and open it, right click and drag on the door and you’ll open it when you release the mouse button. You can talk to people like this also. To initiate combat mode, have a weapon in your hand and left click on the weapon to use it.
What I really appreciate about the mouse control in this game is that it’s analog and gives far more options than wasd does. The learning curve however may not be something you can get away with now we have standards for this sort of game.
One of the numerous manuals that came with the game was a sort of starting walkthrough from what I recall. Looks like that’s included in your link – the manual had some pictures to go along with it which might help.
You actually have a slot for the torch that doesn’t require a hand at the top left. I think it pops into there if you light it from your pack. It’s never occurred to me until now that it doesn’t actually use up a hand to carry it there. Must be a head torch I suppose…