Thermostat wiring question (Boiler and Air Handler)

Question

Because I have both a boiler (in the basement) and A/C air handler (attic), I have two bundles of wires at my thermostat. One bundle has wires that connect to RH and G, and the other has W and Y. Also, btw one wire in each bundle (both blue) are wire nutted together. I went to the attic to check the connections at the A/C air handler and R and G are wired, so it seems I know which bundle goes where. Also the blue wire is connected to the X terminal. I haven't checked the circuit board at the Boiler, yet.

So my question(s). Like many, I'd like to install a Wifi thermostat and want a C wire connection. I have spare wires in each bundle. There's empty W1, W2 and Y terminals at the air handler, but no C terminal. Can any of these be used as C? Also, why are the blue wires wire nutted? I've attached a picture of the schematic at the air handler (happy to take additional pics as needed). Also I don't understand why the Y terminal isn't used at A/C air handler, and rather the Y wire from the thermostat seems to go to the boiler.!!?

EDIT: I've now added pics of the boiler schematic and board (second and third pics, respectively). I have some theories on the whole configuration below pics.

Here are a few theories: 1) The W wire from thermostat links to T terminal on boiler as power, with the X wire from the airhandler providing the return via the TV terminal at boiler to X terminal at Air handler 2) The Y terminal is not used at Air handler because it's not linked to anything. Instead the Y wire runs from thermostat down to basement, using the thermostat bundle that's heading in that direction then splitting off to AC condenser outside, likely also with the X wire from the air handler as common. 3) The X wires (blue) are wire nutted since they are not needed to power the thermostat and instead provides the common loop for condenser (and boiler?) 4) Leaving everything as is, I can take a spare wire at the Air handler, connect it also to the X terminal and connect that to C at the new thermostat.

Answer 1

I believe in this case the X terminal is what is now called the C terminal - it's the "other side of the transformer connected to the R terminal", without anything switched from it.

i.e. power flowing from R to X won't cause something else to happen at your air handler, whereas power flowing from R to G or R to W1 will, and no power will flow from R to Y or R to W2 since they are not connected at all.

This is by no means the "only meaning" of X in thermostat wiring, - in some cases it evidently is used for auxiliary heat. However, it is known to be equivalent to C for some makes and eras, and the schematic shows that to be the case for your air handler.