If what you're saying is true, then every time an ETH is bought or sold for BTC, the volume is attributed on both sides of the ledger (i.e. it's ETH volume and it's also BTC volume). That's not how it works. CoinMarketCap treats Bitcoin as the primary crypto currency
I'm not sure how CoinMarketCap is designed to works. What I described is the correct way of how volume should be accounted for in general. Volume doesn't care if it is a sell or buy transaction. All BTC transactions are accounted in BTC and all ETH tranactions are accounted for in ETH. It doesn't matter if they are sell or buy orders.
Yes, true. But currently when volume is spoken of in this crypto realm (and in essence how CoinMarketCap is operating) BTC volume is only ever buy/sell against FIAT. However ETH, DASH, DOGE, LTC, etc volume is a combination of buy/sell against BTC and FIAT (and the corresponding BTC buy/sell that naturally occurs when buying or selling the Alt via BTC isn't then attributed to BTC's volume).