I thought color negatives had a much wider exposure latitude than slide film. At least, that's what I've always been told. I'm thinking the trick isn't exactly poor technique. You are just trying to capture an image with a brightness range beyond your films' capabilities. One of your mistakes was trying to get the entire range on inherently high-contrast Velvia and Provia slides. To me, it looks like you overcompensated for the brightness of the snow and got quite a bit of overexposure. I enjoy Provia and Velvia, but they seem to be the wrong choice for a scene like this if you want the full brightness range. These films are high-contrast and have incredible color saturation, but your scene is also naturally high-contrast and not exactly colorful. It almost looks black and white with a little blue in the sky and some green in the trees.