The upside is that when you've fixed one area, you only need to do some basic stuff and lift with proper form to maintain it. I do't do nearly as much for the hips and shoulder these days. Now I just need to get back on it to fix my supinated feet (collapsed ankles) and bad external rotation of the other arm .
And yeah, the couch stretch hurts a lot and is really tough when you start doing it. I've shown it to some friends and they nearly cried and gave up about a minute in. Just remember to keep the front legs shin vertical, neutral spine and squeeze your glutes hard to keep the spine in that position. My top tip for doing boring static mobility work like the couch stretch, bottom squat sits or external hip mobility flossing, is to distract yourself. I can easily 10+ minutes in a position if I'm reading a magazine or watching a series or whatever, whereas I would stop about 2-3 minutes in otherwise due to pain and it being boring.
Elbows in the squat: I think Rippetoe is one of the only coaches that advocate the elbows pointing back thing, and as you know his way of teaching the squat leaves is not the best.
Elbows back can make you lean forward and round your upper back. Elbows under and driving forward when you come up of the hole prevents that.
Not a huge fan of these, I found most of them short and not that challenging. It might be a good place to start but I think there are better videos out there if you don't mind paying or pirating.
Trying to do ashtanga series I is a lot more engaging, in my opinion. Start just by trying to get the first ~15 minutes correctly and then slowly work up to the whole thing.