ADAR4002 (EVAL-ADAR4002) SPI readback: no SDO activity via FTDI/pyftdi

What do you have the MODE pin set to?  It should be Low if you want to use the ADAR4002 in SPI mode.

The Mode pin is already low, but no response.

How are you connecting your SPI signal to the ADAR4002 evaluation board? Are you connecting via the SMAs, P1 header, or the P10 header?  

P10 has a Do Not Install resistor R6 on the SDO line.  You would need to populate this with a 0 Ohm resistor to readback correct when using P10.

I am using the P10 header as described in the manual.
I am measuring at the pads of resistors R4, R90, and R91 that face toward the center of the board.
The signals are present and at 1 V logic levels. At R92 it remains low.
My protocol analyzer in the MSO also shows the hex values I am sending.

This is really weird behavior.  Have you tried writing to and reading from other registers?  For example, the scratch pad register 0xA? 

Also, have you tried controlling the evaluation board with an SDP board and the ACE plugin?  

You are driving into P10, so you are using the level translators.  

• How do you have P8 configured?  This configures the high-side supply of the level translators (VL in the schematic).  With the SDP attached, P8 should have a jumper shorting Pins 1 and 2.  If you do not have the SDP connected, you would need to supply 3.3V (or whatever your high-side logic level is) to the VIO_3P test point.  
• Alternatively, you can short Pins 2 and 3 on P8, and drive the high-side level shifter voltage on Pin 5 of P10 (V_LS in the schematic).