Update: I've been to the ranch twice in the last week, installing the ocean terrace station and 2 more repeaters and tinkering with things to get them working.
I've learned a lot about repeater chains and long range repeaters.
The big thing that Davis never addresses directly is time. As anyone with a Wireless VP2 or Vue knows, it can take 15 minutes for the console to sync up with the transmissions from the ISS. However, that also applies to every repeater in the chain, in order. So, Station 5 on the ocean terrace, is repeated by Repeater A, then Repeater B, then Repeater C, and then the console. If it takes 15 minutes for A to find the ISS, B to find the repeated signal from A, C to find B and the console to find C, that's a potential hour before everything syncs up.
And, I found a source of Davis interference. If you look at the image above (I need to redo that with more accurate 5 and Repeater 1 locations, and relable the repeaters to the Davis standard letters), you'll see a road running N/S of Station 1, and a white terminus just north of the station. That's a Verizon cell site.
I moved the standard repeater from 3 to 1, and installed a long range repeater with 2 Yagis at 3, to better pick up 1 and Repeater 2, and drill the repeated signals right into the barn. After correcting a mistake in configuration on my part, nothing worked from that repeater to the Barn. After a week of thinking about is, and an hour of troubleshooting, including about an hour total on the phone with Davis tech support, it started to dawn on me. In this county, Verizon is limited to the 800 MHz band. That's close enough to the 900 MHz spread spectrum that Davis uses, that combined with the 11dB receive yagi and the proximity of the cell site to Station 1, it was enough for the cell site transmitter to swamp the receiver in the repeater. The site is within the beam width of the Yagi. I proved this by moving the the Yagi off to the left of directly at Station 1 and it started kinda working. And, since I was in that sector of the cell site, it stopped working every time I made a call and the transmitter came on.
To fix it, at least for now, I switched the regular repeater at 1 and the long range one at 3. Everything started working! There are still some data dropouts presumably caused by the cell site, but now I can figure out how to address them; by moving a site, changing antennas, or adding a 902-928 MHz band pass filter.
This is looking from the top of the bluff at the Repeater 1/A site to the Station 5 ocean terrace:
A little zoom shows the pole set in the clearing just off the end of the T intersection.
Here's the station installed:
Repeater 1/A on the pole at the top of the bluff, 500' above the station, RX Yagi aimed at the station 500' below and 1,500' away, TX antenna aimed toward Repeater 2/B.
Here are the distances, as measured on Google maps: The actual distances are probably longer, as I don't believe GM accounts for elevation change.
Station 5 to console via 4 hops- 8,124 feet
Station 1 to console via 2 hops- 4,654 feet
Station 2 to console via 2 hops- 2,974 feet