Mari's Chinese name is Liu Hiu (Liu is the family name) "Hui" translates to elegant flower, Marigolds are elegant flowers so Mari shortened marigold to Mari for her English name. Many Chinese people use English nicknames when talking with their friends even if everyone is Chinese because using your Chinese name can sound quite formal, like when your parents shout your full name just before telling you off.
it started out as a joke but it stuck now they all call him Dave and he wouldn't want it any other way.
Yes, Tsung Tsung meaning pair in the local dialect, his first Chinese name was Dandan but Mari's mother said it sounded like the Chinese word for alone, which is bad luck, so she suggested a new one.