I have already replied to your post on the other forum.
But here my answers again (it seems that you have already solved the inventory problem, since you didn't post it here):
I would do it like this:
Create a hotspot for the door, add the picture of the door to it (General Tab -> Hotspot Appearance)
Go to the variables tab, and set it so that the hotspot is only enabled if the variable door=1 (you need to create a new integer variable called door).
Create a second hotspot on top of the first hotspot, and leave it blank. Click the "Manage inventory" link on the Frame Editor. Drag the item onto that hotspot. Select the "remove the dragged item..." on the Action tab.
Go to the Variables tab, and set it so that the variable door becomes the value 1.
Then the door should show when you come back to the screen, and not disappear.
No, currently the inventory at the top of the screen is the standard.
With the full version of AM, and a lot of scripting, it would be possible to create a custom inventory, but would be a lot of work. I'd suggest you to use the standard inventory at the top of the screen