1: I would say that it's very much a doctor strange movie. It's an exploration of the character, how even across the multiverse he's always similar. He always wants to "hold the knife", making risky plays to get the best outcome. He never gets in a relationship with Christine, and he almost always gets tempted by darkness. Even the What If Doctor Strange is the exact same. Narratively, he's also pretty motivated. He's the one to take out Shuma in the beginning, and teaches Chavez how to multiverse-hop at the end. The only thing he isn't particularly involved in is stopping Wanda, but I don't really think that he needed to be there. It continues the theme of Dormammu, where he stops an antagonist much stronger than himself via outsmarting them, not via simple strength. Plus, it's a powerful character moment for her, i don't think a zombie doctor strange sitting in the corner would make it better. Wanda is a very important character in the movie, but she's definitely the antagonist. This isn't even the marvel movie that gives the most attention to it's villain, it has to be tied with Loki, Zemo, and Killmonger (and behind Wenwu and Thanos))
2: Wanda is literally stronger than Strange, unless he gets the book a fight between the two would end in him dead. It's also a chase movie, with Wanda essentially being a slasher. Why does Ellen Ripley run away from the Alien?
3: Wanda wasn't portrayed sympathetically after her literal first scene. And, in that scene, calling on her for help makes sense because she's the one with the most experience with magic. Yeah, she did ****ty stuff, but she appeared to have righted her wrongs best she could, and is looking for redemption. Why would Strange go up to her and say "You brainwashed a town. **** you. Anyways I need your help...", it's literally in Strange's best interests to at least pretend not to care. Besides, in Ragnarok he acted at least somewhat amicable with Loki, who did far worse (and considering Strange is a New Yorker would hold a personal grudge as well), so there's a precedent.
4: First off Wanda is being corrupted by the literal Book of the Damned, which might impact her logical thinking a touch. Aside from that, she doesn't want just any kids, she wants Billy and Tommy, her sons. If she went to a sperm donor, there's an incalculably low chance that she gets Billy and Tommy back (it's also very likely she wouldn't get twins at all, and a coin toss if she gets a son or a daughter). As for her not going to a universe where her kids are orphans... no argument there, but then we go back to the Darkhold.
5: Yeah it's pretty silly, but it's not even the most plot-convenient random animal. Thank you Radioactive spider and Endgame rat.
6: I feel like only exploring a few universes was actually the right call. Earth-838 feels like a fully lived in alternate universe, with it's own history and culture. The destroyed Earth at the end goes back to the first point, showing what exactly happens when a Doctor Strange grows too Machiavellian and dark. I swear, people wanted Doctor Strange to go to a new universe and exclaim "holy **** it's (insert random marvel character here)" on loop for three hours. We have plenty of multiverse projects in the pipeline (What If S2, Loki S2, Ant-Man 3, Marvel Zombies, Deadpool 3) with more inevitably happening after, no need to rush through it all now.