I am a bit confused over the NACHI Pre-Licensing Curriculum’s detailed emphasis on the required 4-wire feed for the remote distribution panel.
The 240V 3-wire feeder to the RDP should be all that is required for a detached structure with a stand alone dual grounding electrode circuit.
Bringing the 4th wire (service equipment ground) to the remote structure, and connecting it to the isolated equipment ground bus in the RDP will only re-create the possibility for the phantom circuit which you are trying to avoid. This phantom circuit is due to possibility of difference in potential between the two earth grounds. The 240V 3-wire feeder does not require a neutral ground since the feed is inherently balanced (just as the SEC is) by the 180deg phase difference of the ungrounded lines. Therefore, the earth ground bonded 3rd wire from the service panel is the equipment ground in the feeder circuit, and should be connected to the isolated neutral bus in the RDP.
Maybe I’ve just had too much coffee this morning…but your 4 wire feeder seems to cancel the possibility of true isolation from the isolated equipment ground bus in the RDP. Then again, I am new at this whole residential electrical thing. I learned all my electrical and plumbing theory working on ships in the US Navy.