You did the right thing by escalating higher to get this resolved to your satisfaction..I would have done it sooner..
This car I believed is still under warranty (based on OP post) so funding is not an issue, dealership still claim for warranty work.. and it was brought back 4 times and was initially serviced therefore mechanic had heard that noise.. just that it was not serviced well enough...
It might not be a safety issue but it is a nuisance to hear that ticking sound for the duration of the journey.... that would be classified as torture. especially for sensitive ear..
End of day. you did what would be beneficial for any E90 owners if they face the same ticking issue.. there is at least a service bulletin they can refer to..
Now enjoy your ride...