The Ritual of the City of Dreams and Phantasms

 Preamble: 
So, in order for the curse of the spider mages to be broken, Camrish must not exist, even to the third odylic sphere. 
 Total Destruction is hard. Total Destruction and perfect recreation is very hard.
The best chance as I see it is a combination of transmutation to something that does not exist, but does not fail to exist, and then translate it via a discontinuity through a different point of existence(the translation being made plausible by the transmutation), then, once the proper moment exists for the curse to be broken, the city is recalled and restored.

The ritual setup and preconditions:

The dream analog of the one who bears The Longing Ring must stand in the city of Parlenth while wearing the Ring of the Dreamers of Camrish.

A stone from each gate of Parlenth must be placed at the 4 cardinal directions of the City of Camrish. A shard of each stone must be placed in the corresponding position in the map room at the heart of Camrish.

All of the Ring of the Dreamers of Camrish on the native existence of the City of Camrish must be present in the map room, at least one of which must be worn by its proper owner. The parts not being worn placed in the heart of the map of the city.

The Powder of dreams must be in the Maproom of Camrish.

A handful of dirt and a bottle of water taken from outside but near the city must be available to the Maproom.

The ritual:

The caster stands at the western gate as the sun rises, and begins the ritual. 

The one who bears the Longing Ring stands at the eastern gate, one step more distant from the city then the caster, and positioned so his shadow will fall over one standing before the eastern gate at noon. 

As the ritual begins, one who rightfully wears the Ring of the Dreamers of Camrish, located in the Maproom of Camrish, takes a pinch of the Powder of Dreams(taken from the Book of Steven Bigby Mandrake) and sprinkles it on the western gate of the map.

The caster circles the city in a counterclockwise direction to reflect the unnatural process of transmographing and banishing the city. He walks and incants at a measured pace, encircling the city, such that he arrives at the eastern gate at noon, where he passes through the shadow of the one who bears the Longing Ring, and continues around the city back to the western gate, just as the sun touches the horizon. As the sun crosses the horizon he completes the final incant to banish the city.

Meanwhile, as he incants in each of the 4 cardinal directions, the one who wears the Ring of the Dreamers of Camrish, who stands in the Maproom of Camrish, takes a pinch of the Powder of Dreams, and sprinkles it on the corresponding gate. When the caster returns to the western gate, the one with the powder takes all that remains and sprinkles it evenly over the entire city.

The city vanishes, a dream untethered by all reality except for the tie of the Ring of the Dreamers of Camrish, binding it to those who dream in another realm.

When the sun next rises, the caster must go to the bearer of the Ring of Longing, who must stand where the east gate once was. The caster must lend his might to the bearer of the Ring of Longing, who must in turn reach out through the ring to the city of Parlenth, there, his analog must both reach back with his will, and forward, through the Ring of the Dreamers of Camrish that the analog must bare, and unify the two directions.  Feeling this pull, the one who stands in the maproom, and wears the Ring of the Dreamers of Camrish must blow away the dust of dreams, and sprinkle the map of the City with a handful of its native soil, and its native water. 

Then, if all goes well, shall the city be restored as the sun clears the horizon.