for 2:
You need to highlight the wall and use mousewheel to scroll through the prefabs.
Indeed. I LOVE this feature

. Dont forget to press ctrl when you mousewheel, to change your wall appearance.
As for del, it works for me, it only removes the door, and it creates two tiny wall sections.
Did you try this root_template ?
05m_PoorHouse_A_FloorTile_A