I think Hub is better; Put a Hub between Switch and destination router or switch or PC.
Plug another PC (with Wireshark installed) into Hub, as you know Hub repeats traffic that came in and sends it to all other ports.
But, yes, it possible connect PC to trunk port; PC must have NIC supporting Vlan 802.1Q. encap.
I don't think I saw a NIC that supports Cisco ISL, so ISL Vlan encap will not be captured.
Yes, that's how you do it. Cisco Catalyst switches support a feature called Switchport Analyzer (SPAN) and Remote Switchport Analyzer (RSPAN). These features are used to collect traffic from certain ports/VLANs that you define and redirect them to a collection station, like a sniffer or an Intrusion Prevention System.
You configure the switch to run SPAN or RSPAN to redirect the traffic to the sniffer, then the sniffer (like Wireshark) is configured to promiscuously listen for all traffic, including the VLAN (ISL or 802.1q) tag.