Cornish meat birds seem small
The cold will affect their weight somewhat.
But here in GA, mine weighed 3+ lbs at 5 weeks.
I feed 24% protein starter for 2 weeks 24 hours a day. After 2 weeks I only give them the amount of feed needed based on their weight.
Starting at week 6 I change to 20% protein starter.
No corn or scratch grains.
And your one that was mounted might have had a mild heart attack!
So 3 days ago,he was barely holding his head up,and made the decision to cull so I could still use the meat. He indeed had heart issues,he had water around his heart. As for the bantam mounting it,it did it to another one,as did a lf roo,so if anything it just made a heart condition worse,not cause it.
Yesterday,I separated mine from the main shed,and made a little coop and a small "bachelor pad" pen area.
I weighed my heaviest looking one, and it was 2lb 9oz, so they are gaining,just not enough yet. I will probably just wait another week or 2 longer to process.