It does seem that partially reflecting mirrors are used, by Sony and many others. So I was wrong there.
As far as sensors go, the current ff Nikon sensor is made by Sony, who happens to be the largest sensor manufacturer in the world. Nikon can buy from others, but there is a clear advantage if you actually make your own. Canon have lagged a little with sensors as they have not used the Sony sensors. It will be interesting to see which sensor Nikon choose next, and when what will happen. It will be fascinating to see what the major brands do over the next few years, as the market is hard to define accurately.

My guess is that Canon will continue at the top, but Nikon will find it tough. Why? (and this is purely my speculation as I'm sure you will all disagree)

Because Canon are dominant in lenses and seem to be focusing on that area. Just look at the new 11-24mm or the 100-400mm lenses. The last lens we bought (just arrived today) was a Canon 24mm TS-E, to be used with a Sony A7R2. Even if the Sony adaptors would take Nikon lenses (which I think they will, very soon), I doubt that I would buy one.
Nikon had a lead in cameras, but are under threat. Their lenses are good, but by many reports, going downhill. And they are small in size compared to Canon, and certainly to Sony.. They may be very creative and survive in the top echelon, but it will be tough. They do have many loyal supporters, but that may not be enough.
Sony are new kids on the block, but have an big opportunity because of new technology (mirrorless) and their size and dominance with sensors and gaming make them a powerful challenge to Nikon. If Nikon couldn't take the lead when they had what was clearly the best camera, what will happen when they don't?