Where is WinRT API to RETRIEVE email? [I know MS are pushing 'share' for use cases normally associated with sending an email]

There is not email specific API and no way for apps to interact directly with other apps (such as email apps) on the local system.

Depending on the server you're talking to they may provide a web API you can connect to.

See Sharing and exchanging data for information on the share contract. It won't be relevant for your scenario though.

--Rob