Okay, I had a look at your file.
1) The offset is because of how the panel cut works: It extracts the base wire of the panel, then rotates it into the XY plane and translates it to the (0,0,0)origin point. So you can have a big assembly of panels, located anywhere in space, all the panel cuts will spawn around the (0,0,0) point. I'm not sure it's the most clever idea, buy it seemed better to me than leaving it under the panel itself, where it could be hard to notice and select.
Ideally, when creating panel cuts from several panels, they should be placed in a nice kind of array... Or even more ideally, already come automatically nested
2) It is possible to add a panel cut to an existing panel sheet. It works like other Arch objects, double-click the sheet to enter edit mode, select the cut, select the "Group" in the task panel, and press the "Add" button.
This system is still a bit clumsy and should be bettered, I should implement drag&dropping from the tree (i think it's possible now in python view providers), and not be forced to click the "Group" after you select an object.