I really hate to sound like a broken record .... but does not Section 90 of the NEC plainly state "this is not a design manual?"
So, how deep should the pipe be buried? Well, as deep as it needs to be when it exits from the protection of the slab, would be a good start. Or, do you plan on changing the trench depth every time you have a change of the surface material? Lots of luck sanding the angled parts.
Or, consider this: what's a likely cause for the pipe being damaged? Besides the gardener using his shovel to look for irrigation lines ... how about frost heave? I don't care how well the pavers are laid .... give them a few frost cycles, and they'll be all over the place. Maybe the angled pavers will now even act like scissors on your pipe.
Whatever depth you decide upon, make it an honest depth. That is, to the top of the pipe, after it's properly bedded in sand - and NOT the rough depth of the trench! This means a 6" burial depth starts off with a 12" ditch.