I think you're right that breeding for temperament makes a huge difference. The breeder I got Sylv from had three strains of bantam Cochin. The millefleurs had been bred to show standard and the roo he used was called 'Snappy' and he really was! He was a biter and i dont think his progeny were very friendly either.
Sylv was very sweet - apart from his fetish for my feet - and was super easy to handle, you could do anything with him except put your hand in the coop when all the girls were in with him. I didn't mind that tiny bit of aggression as he was just doing his job protecting the girls. Ferdy doesn't have a nasty bone in his body. He was handled from day one and I did everything you aren't meant to with a young roo - letting him get on my shoulders, head etc. It looks like Stanley is shaping up to be a little mini-me. Again, I've handled him from hatching. I've never forced them to do anything that frightened them or made them uncomfortable.
The girls have been a little less confident and friendly than the boys but they're from a different breeder. They are handled just the same but I don't think they're from such a friendly line. So genes are definitely important! Having said that, Clara has gone from quite nervous to a real cuddlebug since she decided she's sleeping in Ferdy's coop. I suspect they learn by example. She saw me putting Ferdy to bed every night with lots of petting and cuddles - which she didn't see when she slept in the big coop. Pru doesn't much like being picked up, so i don't unless i have to. She's very food orientated though so I use that to my advantage. She'll never be comfortable with hugs and I accept that.
Sylv's genes are obviously more dominant than Clara's and Pru's.
Having said that, Penelope was an absolute little witch when she was a teenager. I once had to get help to remove her from my lip. She bit it and clung on for grim death! She would bite at any opportunity. Once she went broody for the first time, her personality completely changed. She now squeals to be picked up and loves a cuddle. I guess Sylv's genes won out in the end!
If I was breeding bantam Cochin, I'd definitely use Ferdy for temperament. He'd be no good for showing due the gold leakage on his feathers, as it's clear that trait isn't going anywhere! I've been told if it wasn't for that, he'd probably do quite well in the 'show ring' as he's very close to the breed standard. IDK why breeders can't breed for both temperament AND looks.