How can I tap into and measure this rf frequency?
I may not answer every one of your questions but I think the following will generally answer your core needs:

With your current frequency counter and its limited sensitivity of -19dBm you are unlikely to be able to measure the transceiver frequency without some modification.  Using a wideband high-gain amplifier such as this Mini-Circuits device may help, but noise and limited power could still be a problem.

A spectrum analyzer would be better choice for measuring the center frequency.  A spectrum analyzer with a properly matched antenna should be able to detect the transceiver signal over-the-air.  Setting the transceiver to a CW tone will ease the spectrum analyzer requirements, otherwise a real-time spectrum analyzer is good choice for UWB detection.  The spectrum analyzer could be connected directly to the 0402 pads of the pie network using a coax probe but it would be necessary to disconnect the chip antenna or otherwise compensate for the additional 50 ohm spectrum analyzer impedance in parallel with the chip antenna.  Another option for direct detection on the PCB would be to use an active probe.