Nic, you've asked a number of questions here, but let me do my best to impart what I know.
Cattle leather does come from various types of cattle, including calves, buffaloes, and zebus. In no sense is cattle leather a byproduct of the meat industry. While it's difficult to get specific numbers, since the cattle industry is widely known for its lack of transparency, articles I have read estimate that cattle leather accounts for about 6-10% of the profit. The purchase of cattle leather increases the demand for cattle, increasing the number of cattle killed.
If you want to include other animals in this discussion of leather, there are animals that are killed mainly for their leather, like ostriches, where about 80% of the bird's value comes from its skin.