Nope, sulfur is insoluble as is the charcoal. The only component of BP that is soluble is the nitrate. That's why people that add bunches of water wind up with weak powder. They either wash away the nitrate or it re-crystalizes when everything dries and they wind up with an "unmixed" powder.
I would think that to check the PH of sulfur, your slurry idea would work as long as you know the PH of the water first.
You shouldn't need to mill the sulfur. Most of us on the dark side simply mill everything together so we get the most homogenous mix. I'm going to assume you're familiar with the proper media to use in your mill.The key to avoiding caking is to dry everything separately first. An oven or roaster works fine but electric skillets do not heat evenly. Surprisingly, charcoal seems to cause the biggest problem. Once dry, you can mill everything with less chance of clumping. If you use dry chemicals, clumping is typically an indication that you've milled the BP long enough.