Re: Engine keeps running after ignition turned off.
I had this a few years back with a Vectra. It kept running (smoothly) until I stalled it.
AA found that live was getting through to ECU with ignition off which was "impossible".
When I took it to a Vauxhall specialist, he didn't sound too surprised and immediately rattled off a few common causes:
faulty light bulb
faulty towbar installation
also asked if I had added anything to electrical system recently. He said he had seen this happen with a faulty bulb and results in a live being fedback (?)
As none of his initial thoughts were the cause, the engine bay wiring loom was taken out and buzzed through on the bench with no fault found.
When the loom was put back in, the car behaved itself an did so until I got rid of it.
The fact yours was running roughly suggests it might not be as simple as the reasons above, but thought I'd share it with you just in case.