Sorry to hear of your health issues. I know on my older A6 the ECU, cluster and injection pump all have to talk to one another to get the car to run.
So you'll definitely need to get the cluster coded. You'll need an auto electrican to help with this.
Safe mode is the car realising that these components aren't talking to one another and the car imobiliser has stopped it working.
Also the sports exhaust shouldn't affect the MOT as long as catalytic converter is fitted. Egr might not fail MOT as long as mot tester can see it fitted.