The Frontier and the Spanish Navara are fitted with the same Dana differentials.
Yes, you can buy different ratio ring and pinions from such as Yukon in the US. Not cheap to buy or install.
In terms of your problem, the slightly lower ratio will not make any noticeable difference, and I...
If it is Spanish, there isn't a code. If Thai then there are several websites that will generate codes using the serial number. Not free, but nowhere near $160.
Or you could buy a cheap radio that will be miles better than the outdated one you have.
You could try the test mode on the instrument cluster to see if the dpf light is enabled.
Press and hold the left cluster button, turn ignition on, wait 10 seconds and release the button.
If the 5th wheel trailer is 9500lbs (4.5 tonnes), it sounds like you are looking at a US import which may be too much too much for the Navara.
There's a few 5th wheeler forums out there who will be your best resource to research it.
You need to upgrade the rear axle to pull a 5th wheeler. It doesn't upgrade the gross weight capacity, just adds about 300kg to the rear axle capacity.
In the UK, SVTech do this.
Be wary about US import 5th wheelers, many are way too heavy for what the Americans call a midsize pickup.
When my EHR failed, the only sign initially was the cruise not setting. Something to do with the maf readings not matching the EGR flow readings. Not sure how the EGR measures gas flow.
It is possible that the delete didn't work?
There's a cable that runs to the ignition switch that inhibits the starter when in park. Just wiggling the lever or pushing it forwards as much as you without breaking it might identify the problem.
The other problem with longer studs is that the front hubs don't come apart to change the studs.
I guess they do come apart, but not without specialist equipment.