1) You don't need any particular status to upgrade with avios - there just needs to be availability and you need to have the avios to do it.

Looking at June now for YYZ-LHR - there's nothing available on the Fridays for 2 people in CW at all. It's possible it might open up but I wouldn't bet my bottom dollar on it - YYZ is a very popular route and is VERY frequently over sold.

2) My recent experience of the 777s on this route were the 300 variety.

This page tracks it http://ftdashboard.net/dest/yyz.htm - and you can match up the plane type to the date. You can see that the arrivals on the Saturday mornings (currently) tend to be 772s.

There's only really 9 772s with old WTP anyway now.

The reason you can't find the 96 is that BA only operate the 96/97 pair in the summer. So you have to go by the current 98/99 pairing.

Note: For greater avios availability - you might consider connecting down to JFK - I've found more availability that way in the past - but if you were booking now with the hope of upgrading later I'd book direct and watch for direct availability rather than booking a connection and hoping.