All true.
Of course. This series barely passes a single episode without preaching the "sisterhood" business which is redundant, and does not serve every story. And you are correct--she's only there for the false "Oh, how can I fight my own loving sister" / "I wont fight my sister" as a set up, when as you pointed out, she has no reason to be there, other than to be a convenient problem for Supergirl.
True. Toe to toe, she's a weak human with no chance against a Kryptonian, but leave it to Berlanti to keep forcing the idea of Alex as some badass.
...but not in Berlanti world, where she needed to assert herself (endless times per episode), and once again, place herself in situations she is not equipped to handle. MM needed to flat out refuse to take Alex with him, but thanks to poor writing, an unprotected (from mind control), non-super powered character creates more trouble for the two individuals most likely to stand a chance against Non.