A sophisticated adversary able to observe traffic can indeed differentiate different users using the same external IP by analyzing TCP sequence numbers and through timing analysis.
They will be able to draw conclusions such as "The person who visited XXX site also visited YYY site".
Since the ISP knows the owner (the one who pays the bills) and street address of the shared IP address police will likely start there. They might simply confiscate the systems in this home and question the inhabitants for more information, i.e. do it the analogue way and not try to somehow trace some internal computer back through the internet.
Run a tool like tracert and you will see where your traffic is lost.
If your connection to the website is routed via VPN, your ISP provider cannot block it.
1) Check the routing when VPN is activated. It may happen, that traffic to some hosts or networks is routed not via VPN, but via your provider.
2) When traffic is really routed via VPN, then the VPN ...