I'll tackle the Betelgeuse issue: In the story there are 3 roles The Main Character, The Love Interest/Sidekick and the Antagonist. Those roles are played by Subaru, Emilia/Rem, and Subaru again. The whole point being that Subaru is his own worst enemy, and by extension the anime obsessed/neet/white knight viewer. Subaru goes through a large character development arc where he starts off really deluded. He believes himself to be a character in a story where he has some sort of magical plot armor super powers. You can see that from the first episode where he tries to use magic to save the girl from getting run over in the road and fails. Subaru is a proxy for a viewer that overindulges in escapism; very common in japan and among anime fans in general. The protagonist is therefor his own attitude about life.

The 'deconstruction' bits of this series are showing that the same escapist personality traits that plagued him in Japan still carry over and eventually drive him to have a mental breakdown. This is where Rem comes in and shakes him out of it, restarting him from zero (aaaahhh the name of the show). From there Subaru really comes out of his shell and grows into a stronger character, however this also means that the Rem confession scene was the point in the series where the main protagonist was defeated.

Meet Betelgeuse. Same protagonist, different face. Betelgeuse is Subaru if Rem had not restarting him from zero (the double meaning of the name of the show). He is obsessive about the witch in the exact same way that Subaru was obsessed with Emilia. It's a one sided love that makes demands. "I've sacrificed everything for you why don't you love me", is a theme repeated when Emilia rejects Subaru and forces him to stay in the capital for mental rehab, and again when Subaru finally gives Betelgeuse the ability to meet with the witch. In both cases the answer is the same, "That's not love, that's obsession. You don't love me, you love the image you've built up of me in your head." Betelgeuse isn't a protagonist that shows up out of nowhere. Betelgeuse is the Subaru that could have been.