Folliar feeding is going to be the quickest way to clear up a deficiency especially if your soil is locked out from too much Calcium. Treat the plant and clean up the soil at the same time plus the results are faster. As far as nute ratios, heavily dilute when foliar feeding. There may be instructions on the bottle, but I would go with less than 1/4 of what's recommended for soil. People foliar feed all the time...it's not going to clog up your stomata, just don't overdo it.
And yes, you need to switch to distilled or RO water until you can isolate the problem. Go to walmart, homedepot,pet store ect you can get 5-1 test strips for $10 that will test PH, Hardness and everything you would want to know. If you use these types of water going foward, you will need to supplement the Cal-mag back in though. I would flush with good water, folliar feed cal-mag (none on the buds) and start a normal regimen with clean water. I have to go to the store every week to buy water, which is a bitch, but you can see one of my grows on RIU that I came close to destroying before I got off tap. Eventually, I'm just going to buy an RO filter for my house. Not all tap is bad, but if you live in an area where the water is super hard, it can mess up your grows.