Add a “C-Wire” to boiler with this device

Based on the second image, I'd say TR2 should be connected to the C terminal. However, for this to work, you'll likely have to swap TH1 and TH2. Since TR1 is associated with TH1, TH1 should be the R wire while TH2 should be the W wire.

The complete circuit should be:
TR1 -> TH1 -> R on thermostat -> W on thermostat -> TH2 -> TR2.

Then you can connect a wire from TR2 to the C terminal on the thermostat.

NOTE: If the transformer is not sized to support the additional load of the thermostat, you could end up blowing a fuse or frying the transformer.

Yes, that should work. I'd prefer to make the splice at a terminal or an existing junction, but you can splice it in the middle of the wire just as well (as long as there's enough slack in the wire).