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no  DC source terminal is << 1 Ohm

no you must examine bandwidth of signal and DC regulator source as expected.  RF signal GHz, and  DC BW MHz

As usual Yes

Computing s21, If Vout no load is 12V on Drain and is  loaded by RF 50 Ohms at 125 mA then Vo = 50*.125A= 6.25 V and Drain RdsOn will be near 50 Ohms with matched Z then attenuation should be 6 dB ~ and any additional attenuation is due to absorption by the DC line filter.  This is slightly impacted by the 301 ohms on R1 by 0.25 dB as I remember and improved by reducing the 301 slightly.

 This design of the demo board is different than previous company owners who used RC filters for every port LF and RF supply & gnd.  So any change affects in both directions. I'm not sure of the assumptions for the design but you must simulate with accurate RLC values for every part to realize more accurately.