It's not an oxymoron. The first ever cryptocurrency was a centralized cryptocurrency. While the centralized aspect was the reason why it failed, the old systems from decades ago were indeed cryptocurrencies. Look up David Chaum.
That is not true, cryptocurrencies do not necessarily have to be decentralised.
Although I suppose that depends on your definition, but then you would be disqualifying something like XRP from being a cryptocurrency. Which people generally agree is.