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Yes, that would work but that would give you no data for the GPS (via the Garmin app for the XT).
Yes, but then no Connected app goodies.
Yes.
You have two issues. How to get the phone calls routed and how to get data to the XT. A (potentially) simpler way to put it is that you would be trying to run two GPS units at once.
So, I would decide which phone function I like more: the XT version or the TFT version. My money is on TFT simply because you can run it via the wunder wheel, no need to touch the screen. To achieve that you would then disable the phone function on the XT. You could still get data to it (the XT) via the Garmin app and having the phone function disabled there is no conflict when calls are routed. You get the best of both with an added benefit: run two GPS routes and compare. Plenty of screen real-estate on the bike to have Connected navigate as well and check against the XT. Connected runs Tom Tom maps, so it would be like having the only two dedicated bike GPS units in existence compete on the same route against one another on the same bike