... tensing his shoulders and arms at the end of his cut.
This is exactly what I was told to do when I practiced with people who do Iwama ryu. And this is what makes the ken bounce at the end of the cut.
I myself learned doing shomen uchi in different way: More soft, not tensing anywhere.
And no wringing movement at the end.
By now I do it a little bit different from what is shown in the video.
It's not aikido, but what gives me an impression or an image of kenjutsu is swordwork like this
As for opening the elbows: We don't open the elbows. We grasp the tsuka in a natural way, with arms completely relaxed. And in no movement we purposely open the elbows. This would be corrected immidiately.
Nevertheless I don't see a way that my elbow touches my face during shomen uchi?
I only practice sword directly wiht my teacher if I don't exactly know what to do. It's too subtle to learn it alone. (Or from videos of one teacher or another.) And what to do depends so deeply on the line of tradition or school it stems from.