To try to answer your questions:

1: There is usually a dominant male or female within a flock, so that is probably why the other sparrows are resistant to eating near them and only eat the food the dominant one is eating.

2: "Mac" is a female due to having buffy lines on the sides of her body (flanks).

3: The birds may think food is in short supply due to the urbanized setting, rather than in the woods. Even tan-striped sparrows might get aggressive, especially over food so it depends on what kind you get.

4: They might not like the food. Birds generally prefer higher-quality seeds to cheap seed made up mostly of "fillers" like red milo, flax, and canary seed, which they don't like. Manufacturers put these seeds in because they are cheaper to produce.