Fuel pump relay location

It may not matter where the relay is located, however, it's a lot easier to troubleshoot if it's close to the item it supports. I'd place the inertia switch right next to the relay. FYI, if the car ever leaves the line violently, inertia switches will sometimes pop.

The basic idea for relays is to take load off switches, etc. No need for a 30 amp switch to trigger a relay. A typical tyco/bosch relay draws under .5 amps.

You have to run a power supply wire back there anyways, what's the issue running a 16-20ga trigger wire?

The oil pressure shut off should not be carrying the power supply, it should likely be in the trigger circuit side.