Here are my observations about the spawn. Hopefully more experienced people can correct/add to this as appropriate:
The various deco items appear in the spawn area at four different times:
First stage - These items appear as soon as the spawn is started. They include some graves, the vats, the levers, the iron maiden and the pedestal.
Second stage - These appear when the first red skull is added. They include the racks, the spikes, some runes and the small banana trees.
Third stage - Appears when the second red candle of the second level is added. Includes the wall decoration, sparkles and soulforge.
Final stage - These appear when the second red candle of the third level is added. Includes the large banana tree, pillory, ore cart, executioner's axe and the remaining runes.
If you run through the spawn as normal, it seems that two items of deco are duplicated and can be taken away. However, when these items appear seems to vary with each spawn. So one spawn may produce a copy at the second and third stage, another may produce them at the first and fourth stage, etc. When an item spawns at the second, third or fourth level, you can get another item by allowing the spawn to regress and then working it back up again. However, if an item did not spawn at a particular stage then one will not spawn at that stage at any time until the spawn is restarted. For instance, if no item appeared at the second stage then it will not produce one there no matter how many times you work the spawn up and down.
It would appear that the item that spawns can be any object which is currently available, not just the items that appeared at that level. So if an items spawns at, say, the third level, it could be any item from amongst the first, second or third level objects. Consequently, getting the higher stage objects is relatively rare.
As a final note, it appears that on occasions you get a double spawn at a particular stage. As I have never had more than two items from a spawn (when working it normally), I would conclude that such a double spawn would preclude any more items from appearing during the rest of the spawn.
There is obviously no way of farming the spawn at the first stage as these items only appears at the very start of the spawn. Farming the second stage objects is fairly easy, and can be done solo with something like a necro. Farming the third and fourth stage items is considerably more difficult and probably requires at least three to five characters, depending on their templates and ability, to push the spawn forward each time (on Siege that is, perhaps fewer would be needed elsewhere).