Yes
Yes. One can use a busbar in different locations to combine the EGC back to the existing ground rod.
No, I'll explain why. Anything that can carry current such as a metal PV frame has to be connected by an EGC to the system ground. This provides a low impedance path back to source of power. This low impedance path is what causes a circuit breaker to trip opening the circuit. The EGC also provides a zero voltage reference point.
An auxiliary ground rod is allowed but reality is it does not provide any ground fault protection because it is not a low impedance path back to source.
Watch the video in this
post.
It still is required to have a ground rod at source (an inverter for example) and the N-G bond. An EGC will still run back to the system grounding as you have a power source that can energize anything metal. The purpose of the EGC is for ground fault protection. Leave the N-G bond in your main panel and do not bond the subpanel.
No, it will not cause interference with the main panel should grid power go down.