Have done many repairable write offs over the years
The reason they would have written your car off is because that dry chemical used to put the fire out is corrosive, it gets into everything as you will see when you start cleaning it, you cant just clean it up with a quick vac and a wipe down with a wet cloth you need to neutralise it.
Here is a link on how to clean it properly and why
https://www.guardianfireprotection.c...isher-residue/
Also repairable write offs that are on the WOVR are able to be insured for full value once repaired and inspected, its only when you sell them people expect them to be cheaper and car yards wont touch them, but 15 year old cars values vary that much its not worth worrying about unless you are expecting top red book value for it.
The wiring is able to be spliced by the right repair method (usually soldered, heat shrink and convoluted tubing), I have done this in the past to burnt deployed airbag connectors this way and most manufacturers sell them with the wiring tails already connected.
A tip for when you do a repairable write off yourself, do a stat declaration stating that you repaired the car yourself to the manufacturers specifications, always good to include a copy of the repair procedure or wiring diagrams in your case, keep the receipt for any parts or the purchase receipt for any donor car including the Vin number of any donor part.
I just use this form and get it signed at the post office
https://www.ag.gov.au/sites/default/...ation-form.pdf
It is stupid but every car has to go for a 4 wheel wheel alignment even if it has nothing to do with the damage you need the print out with all measurements before and after with all after measurements being in spec along with the receipt.
Seeing your car has a burnt seat and the airbag wiring was damaged they may require a airbag test certificate from Ford, they will not accept it from anyone else (no scan result from auto elec)
Hope this info helps if you have never done a repairable write off before it can catch you out and it makes it hard once you have done everything to go back and jump through hoops to get it passed