Time delta from previous captured frame may differ from previous displayed frame when a display filter is applied, whereby the display filter hides an intermediate frame from the list.
The time delta from previous captured frame could be seen as the packet inter arrival time, as far as the time stamping accuracy from the capturing entity goes. Usually the captured frame travels up into the network stack a bit before being timestamped, so it might deviate a little from the on the wire time.
As per answer 2, the time delta from previous captured frame is usually sufficient.
Time delta from previous captured frame is the best Wireshark can do, for timing accuracy it depends on the OS networking stack doing the time stamping , or the time stamping hardware capture device, if used.
