Well, we know we're not getting the 135 Volts to power up the Horizontal circuit...and I decided to look at WHY the G2 board "supposedly" works but doesn't deliver the 135 Volts to the D board.
So last night, after I got off work I pondered your issue...then remembered I had a training manual for your TV model.
That, at least gives us a different frame of reference on how you start your TV up.
I'm still studying this "start up design" right now, but it looks like the G2/Q Box does fire up the Main power circuit which provides the 135 V to the Horiz Drive circuit...that means the MAIN power circuit has to be dead, and I think that was one of 2 circuits you replaced the IC in.
Sony KDP-57WS550 AX1 Chassis
what did you have at CN6900 before and after you pressed the power button?
My reply I had 120v dc before and about 347v dc after pressing the power button
I replaced ICs 6400 and 8002 and reinstalled the D board
IC6900 powers up IC6400 which powers up the 135V
IC6900 on the G2 board sends power to CN6901 and/or CN6401on the D board
I have 0 volts both at the 135V locations on both sides of Q5201.
So we have to find that 135 Volt SUPPLY problem
Picture #1 - Power supply Block, Startup Description
Picture #2 - 135V Primary Power circuit - IC6400 primary test points highlighted in yellow
When troubleshooting the AX-1 chassis power supply system, keep this sequential DC Start voltage structure in
mind. For example, if the HV power supply fails, it may be actually a failure of the Primary Power Supply, which
supplies the DC Start voltage to the HV Power Supply, or the problem could be further up in the system in the QBox
Power Supply, which supplies the DC Start voltage to the Primary Power Supply.