Range Rover L322 Tdv8 2012 no crank

[devilsadvocate]

Hi, it's a long story so I'll break it down in parts.
- Car had a flat battery, door Handle unlock barrel is spoilt. Car was inaccessible to replace or jump the battery
- A stupid YouTube video suggested powering the side repeater lamp for car to get power to unlock it with remote
-Without thinking tried doing it. Didn't work. Finally hooked up external battery with positive lead on the starter motor.

- Car Unlocks. Yay! Hook up a fresh brand new battery. Engine won't turn over. After I press brake pedal and push the start button, car just switches off.

-Bonus point for repeater lamp not working anymore. Probably shorted something

-Tried switching on the ignition, powered the starter motor solenoid from external battery. Car starts perfect. Doesn't sound like a security issue.

- Swapped Starter Relay. No difference.
- Brakes are also unusually spongy. They go down as if there is a problem with the Master Cylinder.

- What could be preventing the starter solenoid from not getting power. I hooked up a diagnostics Launch X431 machine. There are no codes in the ECU, Transmission or ABS. Although there is a repeater lamp issue in BCM and no communication with ABS module in the SRS module.

[devilsadvocate]

There's a small update. When I go to the BCM module from my X431 and actuate the crank request signal, the vehicle starts while pressing the brake pedal. There is something which is preventing it from starting from the push button. Do I look for a new push button? Please help on this. Thanx