Answer

The integrated 2.4 GHz PA is a differential PA and there is an on-chip balun as well for the 2.4 GHz part which does the differential to single-ended conversion, but does not do the impedance match to 50 ohms. That is why a simple external (single-ended to single-ended) L-C ladder-structured matching network is required for impedance matching and filtering purposes. The 50-ohm point after the external L-C structure is basically needed for conducted validation capabilities. At the output power level of +10dBm or below the 2-element L-C matching network is recommended, but above +10dBm the 4-element L-C-L-C network is suggested in order to have enough margins at the harmonics.  
Of course, a single-ended antenna can directly be designed at the chip pin which would require to have approx. 20+j10 ohms antenna impedance, but this wouldn't allow having separate tuning possibilities for the conducted spectrum including the harmonic filtering.

More info can be found in the application note AN930.

Answer

For the pre-built NCP images we include in EmberZNet releases for EFR32MG1P132F256GM48 and EFR32MG1P232F256GM48 variants (included in our Mighty Gecko Wireless SoC Starter Kit):

NCP-UART:

• PA0 – TXD
• PA1 – RXD 
• PA2 – CTS
• PA3 – RTS 

NCP-SPI:

• PC6 – MOSI 
• PC7 – MISO 
• PC8 – SCLK
• PC9 – nCS
• PD10 – nHOST_INT
• PD11 – nWAKE

To access these signals on the WSTK board, connect the external host to the Expansion Header pins according to the table "Expansion Header Pinout" in UG151.