Mazda 3 Diesel Problems
Hyundai i30: Would a diesel be suitable for Uber use?
Diesels deliver their efficiency benefits best when on the open road, but it would still be more convenient for Uber use because they'd need to be refuelled less often than the petrol. The only thing I'd be concerned about re inner-city use would be the diesel particulate filter. If you only drive around the city, this probably wouldn't give the DPF the chance to cycle itself every now and again, which can lead to very costly repairs. If you do head out onto the motorway occasionally, this wouldn't be an issue. The new i30 is an excellent small hatch though, and I wouldn't hesitate recommending it.
Mazda 3 2008: Clogged DPF
I don't think you have a claim against Mazda at all; they rejected your claims because you hadn't followed the recommended servicing schedule and the DPF wasn't reset. They have every right to reject your claim when the filter became clogged. The sales people were not to know you would be doing some of the servicing yourself when they sold you the car, the service people had no knowledge of what you were doing, the parts people who sold you the oils and filters didn't know and couldn't be expected to know, so I don't believe you could blame anyone else for having a problem.