I'm sure someone will be able to confirm it, but I'm pretty sure that the News Channel is the sustaining output for most, if not all of the opts.
yup, that's right. It's the case for the main bulletins, but not the shorter ones. I don't know if it's directly related to BBC One HD, but certainly started at around the same time - last November, I think.
It's also got the added benefit that if a region fails to opt, they don't get irrelevant London news - the nations, in particular, are keen on this change.
There's also a 30 second weather update on the sustaining feed during the regional part of the 8pm bulletin.
(if, say, a region's studio was working, but their presenter didn't turn up, they could arrange for a neighbouring region to be fed down the line and opt with that instead - and something similar can be done in the coding & mux domain)