Welcome!
I'll let the guys chime in here on some questions you have.... as far as warranty, tuning it will likely void it IF something happen. But, the increase in power is nice.
https://www.ecodieselsettlement.com/
But, you'd have to put the truck back to stock to get the payment. Can't put the AEM tune on the truck with no DPF..
Welcome! I would ask what tuning they are using, as most diesel shops just resell big name tuning out there and their upcharge. Usually tuning should be about the same price, and you're just paying the labor for the install + hard parts.
Welcome..
ETC could mean a number of things, more than likely it's emissions related. I'd recommend reading around the forum as we have plenty of threads around this issue that can maybe point you in the right direction.
Welcome! The dealer is correct in this case. You need to drive a good bit weekly at highway speeds to allow the DPF to heat up and burn out the matter that's built up during your city driving.
Hope you enjoy it here, we've got plenty of information for you to read up on!