I ferment most of my wines in the 70s. The best way to know the best fermenting temperature is to go to the manufacturer's website, but I think cotes des blanc likes mid 70s, so I think you're too cold.
I never heard of keeping yeast cold to inhibit it fermenting dry. I think it will ferment dry and then it's best backsweetened if you want it sweetened. I don't think stressing the yeast will provide you the sweetness you're looking for.
Broken Leg Brewery
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