The problem appears to be the drop you are assuming is in the feeder from the station. The current does not flow on the neutral only and the circuit impedance is not just the conductor.
I modeled a quick substation and added a 10 mile feeder on a 25 kV 3-phase line with #336 ACSR phase conductors and a #4/0 ACSR neutral. I then dropped a single-phase resistive load from phase A to neutral at the end (~ 20 A). I used 100 ohm-meter soil, 0.05 mile spans, and 25 ohm grounds at each pole (but no additional secondary grounds). I put no ground at the load and got:
0.0 miles out (at substation):
Ia = 19.79A<151.44?
Vn = 2.211V<157.01?
In = 9.79A<-25.44?
Ie = 10.13A<-36.70?
2.5 miles out:
Ia = 19.80A<150.77?
Vn = 174.5mV<125.50?
In = 8.293A<-20.80?
Ie = 11.77A<-38.48?
5.0 miles out:
Ia = 19.82A<150.09?
Vn = 111.7mV<58.62?
In = 8.276A<-19.63?
Ie = 11.85A<-39.26?
7.5 miles out:
Ia = 19.84A<149.43?
Vn = 1.714V<-97.12?
In = 8.164A<-27.55?
Ie = 11.72A<-33.78?
10.0 miles out (at load):
Ia = 19.87A<148.76?
Vn = 16.6V<-3.97?
In = 19.87A<-31.24?
Ie = 2.398pA<96.98?
Load:
Ia = 19.87A<-31.24?
In = 19.87A<148.76?
I added a 5 ohm ground at the load and got:
0.0 miles out (at substation):
Ia = 19.79A<151.44?
Vn = 2.211V<157.02?
In = 9.79A<-25.43?
Ie = 10.13A<-36.70?
2.5 miles out:
Ia = 19.81A<150.77?
Vn = 170.6mV<125.99?
In = 8.294A<-20.82?
Ie = 11.77A<-38.46?
5.0 miles out:
Ia = 19.82A<150.10?
Vn = 143.5mV<48.09?
In = 8.305A<-19.62?
Ie = 11.83A<-39.30?
7.5 miles out:
Ia = 19.85A<149.43?
Vn = 1.381V<-104.60?
In = 8.111A<-25.67?
Ie = 11.81A<-35.02?
10.0 miles out (at load):
Ia = 19.87A<148.77?
Vn = 13.34V<-11.65?
In = 17.38A<-34.18?
Ie = 2.668A<-11.65?
Load:
Ia = 19.87A<-31.23?
In = 19.87A<148.77?
5 ohm ground:
Vn = 13.34V<-11.65?
In = 2.668A<-11.65?
Ve = 0
Ie = 2.668<168.35?
I then changed the spans to 0.125 miles (less grounds/mile) and got:
0.0 miles out (at substation):
Ia = 19.78A<151.44?
Vn = 2.336V<155.77?
In = 9.279A<-23.35?
Ie = 10.70A<-37.93?
2.5 miles out:
Ia = 19.80A<150.77?
Vn = 748.9mV<145.77?
In = 8.278A<-21.44?
Ie = 11.75A<-38.02?
5.0 miles out:
Ia = 19.82A<150.10?
Vn = 694.3mV<-120.78?
In = 7.965A<-22.28?
Ie = 12.04A<-37.09?
7.5 miles out:
Ia = 19.84A<149.43?
Vn = 4.666V<-72.22?
In = 8.784A<-32.93?
Ie = 11.08A<-29.86?
10.0 miles out (at load):
Ia = 19.86A<148.77?
Vn = 19.05V<-15.08?
In = 16.24A<-34.98?
Ie = 3.809A<-15.08?
Load:
Ia = 19.86A<-31.23?
In = 19.86A<148.77?
5 ohm ground:
Vn = 19.05V<-15.08?
In = 3.809A<-15.08?
Ve = 0
Ie = 3.809<164.92?
I then removed the ground at the load and got:
0.0 miles out (at substation):
Ia = 19.78A<151.43?
Vn = 2.340V<155.70?
In = 9.259A<-23.27?
Ie = 10.72A<-38.00?
2.5 miles out:
Ia = 19.80A<150.76?
Vn = 813.0mV<153.79?
In = 8.224A<-21.52?
Ie = 11.80A<-37.92?
5.0 miles out:
Ia = 19.81A<150.09?
Vn = 1.174V<-110.45?
In = 7.894A<-23.82?
Ie = 12.05A<-36.05?
7.5 miles out:
Ia = 19.84A<149.42?
Vn = 6.516V<-61.65?
In = 9.500A<-36.53?
Ie = 10.43A<-26.40?
10.0 miles out (at load):
Ia = 19.86A<148.75?
Vn = 26.66V<-4.08?
In = 19.86A<-31.25?
Ie = 3.038pA<33.75?
Load:
Ia = 19.86A<-31.25?
In = 19.86A<148.75?
So you can see the NEV depends on where you are located on the feeder. At the end of the feeder, adding the ground dropped the NEV (works up to a point). Less pole grounds can increase the NEV at the end. It also matters where the loads are at in the system.
With many parallel grounds on the system, and with the loads reasonably balanced, the 2V to 8V you wondered about is reasonable. If it is way higher than that, there might be a problem on the system.