Well.. that is almost it.
The door has to be built and saved as a map entity before it can be assigned a code, what you have at this point is a block that won't move.
So with your technique what you must do now is select your door and get it into a new level. You can copy the door and paste it into a new level...this sometimes causes problems but usually works well and is safe. The way of the master game builder is to select the door and then delete the rest of the level Be very careful to save this under a completely different name or you will lose your entire level!
Once the door is in a new level position it to turn along the y axis and determine where the origin is. Build as a map entity and save. Open your original level and import the map entity. position door and assign code. the code already has a trigger and you can assign an external trigger and a key.
The way I prefer to do it is build all my doors first. Then I build the walls around them. Doors are one of the most important elements of an indoor level so I build the rooms around the doors.

Also, if I understand what you are doing you are ending up with two sets of walls were your rooms are joined and punching a hole through the two walls. You need to eliminate one of those walls, it serves no purpose.

Generally speaking there is no way to have different textures on one block face. A block can have six different textures...one for each face. So if you wanted to have a hallway floor with a strip of carpet down the center and tiles on the side you would probably use three blocks two on the sides with a tile texture, and one down the center with a carpet texture. You can get around this by using a sprite... one block with a tile texture and carpet sprite down the center. You have to decide if this is an efficient way to go.
The other perhaps best way is to devise a tillable texture that gives the appearance of having a carpet down the center but is all one texture.
It all determines how the level looks and the speed at which it will run.