1. It depends on the impurity and the emulsion. And no, you cannot tell before hand. We used 99.99+ at EK, but I use straight Analytical grade which IIRC is a bit better than 99.8, but in any event with the work we are doing you will see speed and fog fluctuations. That is about the size of it. In real production, a variation of 1/2 stop is a disaster, but I suppose that we can live with that. Right?
2. Cadmium is used with Ag/Cl/Br emulsions to adjust curve shape. It has little if any effect on curve shape on other emulsions but there is some toning effect. So that is the primary use outside of Ag/Cl/Br emulsions. Cd can be added either during the precipitation or afterwards just before coating and after washing. The salt used depends on the emulsion. If you make an Ag/Cl/Br, you may not want to add a halide salt but rather may want to use the nitrate. OTOH, if added during pptn. you may want to use the corresponding best halide salt and add it with the alkali metal salt. Cd is used in massive quantity in some cases and in tiny quantities in others. It is VERY toxic. Have fun.
A side note to those here who poo-poo the idea that Photo Engineers are useful and denigrate our "science".....I ask them, could they answer these questions? You all know who you are! I hope you read this!