Hi guys

My mk6 zetec s won't start. 
Here's what's happened / gone wrong.
- Decat around 7 months ago, fuel has gone from 50mpg to honestly not even 20. its shocking.  apparently ecu wasnt modifeid so not sure if it has anything to do with this. Cat has now been put back on and the problem is still there.
- within the last couple of weeks the lights on my car have randomly started flashing themselves while i'm driving.
- 04/10/23. We are driving on the motorway, and the lights start flashing. My dash goes completely blank, no lights, no speedo, car is still moving. I turn key off and on and everything comes back, but then all warning lights come on and a messaging saying 'power steering fault' shows up with the airbag light too.
I panic and pull over. We call the AA who tow us up the road to the next layby. They look at the car and the alternator is giving 17V. They want 110£ to tow me 3 miles so I politely decline. The guy tells us we can try unplugging the alternator and running just on the battery to get home. He unplugs the alternator. We make it home and the car seems fine, no warning lights.
My brother then reconnects the alternator. He touches something on either the battery or alternator and it begins smoking. The car no longer starts. Not a single light shows when you turn the key. You cant even hear the small humming sound.

Does anyone have any Idea of what could be causing this? Some people i've spoken to said it is literally just the case of changing the battery. Others are saying the car is broken beyond repair. I really love my car and want to save it if i can. Please help
 

Blown the main fuse or burnt the modules out when he shorted the power, start at the battery to check cables and check if anything is loose. 

Then check all fuses

After that, suspect some blown modules, probably GEM (BCM) or PCM (Engine ECU)