I agree, cull him and go with the nice older rooster. Your current boy is likely to get worse and if you hatch chicks from the current rooster, they might inherit his aggressive tendencies
As for what's wrong, I'd reach out to your state poultry extension for testing options. We can speculate all day, but testing will give you definite answers
Also again, don't blame yourself. These things happen and illness can strike a 12 week old just as easily as a 7 week old, waiting...
It's just kahm yeast, harmless and completely fine to feed the birds (although it can smell a bit). Feed it to the birds and start over if it bothers you, otherwise just ignore it
Utter nonsense. If that's true, then how would chicks with a broody hen survive? She takes them with her off the nest when they are only a few days old. She sure isn't waiting until they are adolescents to bring them outside (and she likely would have weened them long before that point). They...
Talkativeness is more of an individual trait than a breed trait. Some hens are talkers and some aren't. The egg song is also going to be loud regardless. I also wouldn't write off standard birds as they don't require that much more room than bantams. I'd also recommend orpingtons and easter...