How to deal with a negative resistor in Z parameters of a passive network?
About the negative resistance and reactance of the transfer impedance in a passive network:
That's no problem. It doesn't claim that a resistor has negative resistance, it claims that the output voltage in port 2 has quite much phase lag when compared to input current in port 1, about 182 degrees at 915 MHz. That's nothing special in transmission lines or complex enough RLC networks.
Your problematic Z21 is well realizable at 915 MHz. You can find the needed circuit in your question just below words &quot;Consider the following circuit&quot; (checked it with a circuit analysis program).
I am sure that at single frequency equivalent passive T-network where Z21 is a single component or one port circuit cannot be built because it would need a negative resistor which needs some amplifying component. But nobody has claimed it can be built, it only obeys the same equations and to be able to do it and to be at the same time a T-network a negative resistor is a must.
BTW the assumption of the existence of certain circuit leads easily to a need of non-existent parts or the assumption itself contains a contradiction. A brutal example: &quot;The resistor that should be inserted in series with a 2 Ohm resistor to get total 1 Ohm resistance&quot;.
A little more complex example: &quot;The  Perfect Future Predictor&quot;.  It has an opamp and a delay line as negative feedback, its transfer function is perfect time shift (=the delay time) from the future to now. It doesn't have nonexistent parts but it's based on false assumption the circuit is stable.