1. The GTX-345 is hardwired to the 530, and using the 530 as a position source for ADS-B in-and-out. Pretty sure this is correct!
Yes.
2. The FS-210 is hardwired to the 530, and is passing data from/to the 530 and to the iPad/FF via Bluetooth: Flight plan, nav data, traffic, weather, all visible on the 530 and FF? Not so sure all that display stuff, other than nav, will make it back to the iPad/FF?
In order for the FS210 to pass the GTX345 traffic, weather, and AHRS data, the two must be interconnected in addition to the connection between the GNS530W and the GTX345. The GTX345 must also be appropriately configured. Although this should be the case, I have helped out with at least 60 installs where the FS210/GTX345 was not done or configured properly, leading to the symptom: I get weather and traffic on my GNS530W, but not on my iPad. Avionics shops don't RTFM, nor do they always do a good job of verifying that all functions work as intended. Do a flight test before you pay the bill with an Avionics technician. Another area of deficiencies in installations I see is calibrating the GTX345 AHRS, this is often overlooked which means that SV won't work on your iPad, or if it does, the attitude is not set to level. I recommend that the aircraft be leveled using jacks IAW the Maintenance manual and that the AHRS be calibrated in this attitude. Calibration is a shop function and once done, may not be adjusted after you leave the shop.
3. There's no need to "pair" the GTX-345 with iPad/FF, since the 530 is talking to iPad/FF via the FS 210. iPad/FF will get all it needs to show me traffic, weather, etc., from the 530, via the FS-210?
Am I at all close?
You are close, the FS210 gets its traffic and weather from the GTX345, not your GNS530W. The GNS530W gets its traffic and weather also from the GTX345. The FS210 is also connected to the GNS530W via a serial port used to exchange flightplan information and GPS information. The FS210 has a built in AHRS and the GTX345 has one as well. Which is provided to the Bluetooth is dependent on if the GTX345 AHRS is setup, in which case it provides the AHRS. I also believe that pressure altitude is provided by both the FS210 and the GTX345, but the GTX345 takes priority.
Finally, the Garmin installation manual calls for its tools to be used for installation. If you or the shop uses ForeFlight for this purpose and run into an issue, Garmin will not provide technical support. By making it an all Garmin issue, they give the avionics shop good support. If it works with the Garmin tools, it will work with ForeFlight.
Normally when there is an FS210 installed, the GTX345 has its Bluetooth disabled and you only pair your iPad with the FS210. There are some circumstances where this is not possible and both the FS210 and the GTX345 Bluetooth are enabled. One that comes to mind is when an Aspen is part of the installation and the avionics shop needs to use the one and only RS422 port to connect traffic and weather to the Aspen. That same port is required to be connected to the FS210. The installer has to choose one or the other. Hooking it to the Aspena and turning on the GTX345 is a solution that works where you can have both.