Art Clay Silver/PMC Silver -- soldering and finishing

1] Yes you can pickle the soldered ART Clay -you have to. The problem is that , because of the porosity, the pickle gets into the metal a bit. I usually make up a bath of warm water and baking soda - this neutralizes the acidic aspects of the pickle. Then there is a lot of drying to do to get the water and the baking soda out of the metal. I have particularly had this problem with hollow forms made over corkclay then fired and soldered - but eventually after several soaks, the baking soda stops bubbling which indicates that the pickle has been neutralized and after that several rinse, soak and dry cycles of fresh water will clean out the baking soda. What happens is that when you dry the piece after the baking soda - it may show these sort of 'frosty' areas - that's the baking soda crystallizing on the surface - so rinse and repeat 'til its all clear
2] Soldering fired ART CLAY to sterling should be fine - the porosity some times soaks up a lot of solder but you should be able to work it out.
3] Re-purposing - every jeweler or metalsmith I know has a collection of metals to do something with. When its steel or copper - we recycle,getting pennies on the pound. You can save the pieces for this purpose and send them into a refiner - these days, the prices are pretty good I think. You can save the bits and sometime take a silver casting class and use them in the class - sawing. cutting and grinding - you may be able to find some little re-fabricated vision in them