Several errors on my C7 RS6 -HELP NEEDED

[porsche997gt3rs]

Hi, I am new here and in quite desperate need of assistance.

My RS6 started first with smelling exhaust in desember, then the errors progressed. It had a tune, and I visited the Audi dealership without them finding any errors except measuring a really high CO emission of about 10.

Then I got it detuned and the exhaust error lamp started to appear. After a while also the EPC light. It started to backfire when idling down a hill or on a strait. Then it startet, or struggling to start. Took 3 times on the start/stop button to make it start. It ran ok for a while, a few days ago it startet to shake violently when driving and I left it one more time at the dealership. Been there several times, in this period. Car is outside waranty, and this might hurt. On friday they said it needed new spark plugs which somehow had been forgotten by the previous owner, but this had no effect on the car. Only affected my wallet. Repairs here in Norway is crazy expensive and I am already out 1700euros, and car is not working.

Error codes are
P030100 misfire on cyl 1/2/4/5/6/7, not 3 and 8
P017500 fuel trim bank 1 and 2 - running rich I think
Exhaust pressure control both sides, they claim this is due to the non-resonated Milltek exhaust.