Try configuring the frame-relay traffic shaping using Modular QoS. Remove the frame-relay traffic shaping command from the interface and configure something like:policy-map TRAFFIC_SHAPEclass class-defaultshape average 15000000service-policy P-QoSmap...

Yes but not a pc/workstation just connecting to a trunk port for basic access.

Yes. It can contribute to cpu usage. Common practice is to just connect the pc to an access port. If you are having issues with broadcast storms you can use the port level storm-control command.

yes if it is a large network and you were to configure multiple trunk ports and then connect a PC to each trunk port.

What would be the purpose of connecting a pc to a trunk port with a native vlan. Why not just connect it to an access port? The PC won't recognize tagged frames and it will drop them and you will just have excess broadcast traffic being sent out that...