I think you're greatly underestimating how much more cumbersome torrenting is even compared to a plugin, especially for "normal" users who are not necessarily tech-savvy.
But back to the point, if Netflix wouldn’t use DRM, it would change absolutely nothing since copyright infringement is still illegal and those DRM protections are completely useless.
If the latter, torrenting is plenty cumbersome enough that if the studios are pushing movie-viewing to "Pay us money or you have to torrent it," they're winning.
Yes, and this functionality has been built into many of the largest torrenting programs out-of-the-box for quite some time now. In the case of µTorrent, it was added in version 3.0 all the way back in 2010.
Obviously, how quickly the stream will buffer depends entirely on the state of the swarm. Popular items will work almost immediately, while particularly unpopular items won't be streamable at all.
Anecdotally, I have personally witnessed my (very nontechnical) friends streaming 4+ GB 1080p ...popular cat videos... that weren't available from Netflix. They did not struggle with the process in the slightest.
Last I checked, the BitTorrent protocol didn't provide packet sorting that would allow for this behavior (by forcing the beginning of the movie's bytestream to be the first data downloaded), so my mistake if the protocol has improved and I was unaware it provided this service.