“12 And all the assembly kept silence”
I like how you removed the rest of the sentence, as if everybody kept dumbfounded silence at Peter. Here’s the whole verse:
Act 15:12 Then all the multitude kept silence, and gave audience to Barnabas and Paul, declaring what miracles and wonders God had wrought among the Gentiles by them.
I don't get how that supports the opposite of what salvation stated. Of course they wouldn't be silent forever after. The text clearly seemed to emphasize the importance of Peter's words by bothering to mention the fact that everyone went silent. If you notice, the Acts only tells us a simple summery of what Paul and Barnabas had to say afterwords. It gives us nothing of the same word for word emphasis of the words of Peter. There was no silence following the accounts of Paul and Barnabas.
Clearly the described narrative sequence emphasizes role of Peter at the council.