I have to disagree on the whitelisting as breaking an implementation of a standard; nothing in a standard like that requires it be universally accessible...you're already authenticating to get the CalDAV data in the first place, for instance. Whitelisting clients is not really different and is quite common, though I'm not sure if Google ever gave an explanation other than "our API is totally awesome", which is not much of one. I definitely do understand reluctance to depend on future support because of reasoning like that, however.
For XMPP, it's the Hangouts app that I was referring to as the "new chat system". Federation for regular google talk was turned back on, but you'll need a third party XMPP client now, I guess. Fortunately they're quite common.