Is WeChat a social network or a messaging platform? I realise it is expanding, but I'd have no problem doing talking business on WhatsApp (which is the closest analogue to what I understand it to be for).

The answer is both. Look at FB - major usage is FB messenger.

Which is a separate app and doesn't leak much to the people you talk to (does it? I don't really use FB or Messenger anymore, I don't remember if you have to be friends to get messages delivered)

Obviously, it leaks loads to Facebook.

Do people have multiple WeChat accounts in China?

You need to provide government ID to unlock many features of WeChat, so that's likely difficult.

I don't know from within China, but from outside China, you can link it to a VoIP account.