To answer the questions
1- given that neither showed anything beyond basic friendship with each other and no attraction towards girls until Volume 9, adding in the OBVIOUS attraction to boys by both with Blake having previously been in a relationship with Adam with heavy implication that it was romantic and most of the series had Blake dating Sun… it stands to reason that writers would continue with what was given.
2- Adam is a sympathetic villain who was shown to have been abused as a child for nothing more than being a Faunus by the SDC, taken in by the White Fang when they were peaceful, learned to fight and protect his fellow Faunus, received praise by others for directly fighting and killing humans which escalated from self defense to outright terrorism… the latter part due to his disagreement with Sienna who did not mind violence but wanted it measured and precise.
3- Weiss was racist in the beginning but learned to overcome that in a very short arc. However, despite how large a role racism played in the plot of RWBY, the writers essentially chose to throw that away and act as if it was never an issue aside from very brief mentions that could be missed.
4- RWBY has a large cast because they created an entire world. Not giving any focus to them would make that world empty and lifeless, like the shadow people they used in Volume One for background crowds. Even when they began giving them faces, it is obvious there was little effort given to that as so many of them are barely recolorings of the main cast: volume two being a prime example as Ozpin gives his speech to the schools and you can count at least a dozen Nora’s given new clothes and hair color. At the same time, given how many characters are vital to the main cast with how often they interact with them, they need some focus. And it’s not like the main cast were written particularly well either.