>>>> turn key to run potions and cluster turns on, door sensor beeps but no injector clatter and no comms at all to OBDII. turn key to start position and all electrical power cuts out?
That sounds like seriously drained batteries or a poor continuity issue.
The first thing, remove one battery negative terminal, then check both batteries voltage. They need to be close to (temp dependent) 12.6 to be relatively charged.
I would look closely at the driver-side positive terminal at the 4ga wire. That wire goes to the Central Control Box, the fuse box. A poor connection there at the higher amp loads of FICM engagement and the other energy grabs like the starter solenoid can cause a serious voltage drop with high resistance. The FICM pulls the higher current of all at the first engagement, but if the voltage drops too far may not engage at all.