Based on the brief description of the shotai shippo technique, that I found - wires are not soldered. They are applied like in traditional cloisonne and sitting on the clear enamel base. In fact shotai shippo is the same as cloisonne, but backing material is later removed. I found some references that nitric acid was used for removing copper.
The question re. counter enamel. In vases - counter enamel may be not necessary, as it is 3D form, but if I would make a flat form - not sure how to do it without counter enamel. If I remove it later - will it impact the piece?