Calculating migration energy barrier in presence of eternal electric field in charged system

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I'd like some help understanding a few issues I've faced. I want to calculate the migration energy barrier for a metal ion (e.g., Cu⁺) in an oxide (e.g., SiO₂) system. Specifically, I want to study how an external electric field affects the migration behavior of metal ions in oxides. To do this, I ran NEB simulations for both a neutral system (no added/removed charge) and a charged system (one fewer electron to make Cu⁺), each in the presence of an external electric field (e.g., 0.1 eV/Å).
For the neutral system, I used the following INCAR tags:
LDIPOL = .TRUE.
IDIPOL = 3
DIPOL = 0.5 0.5 0.5
EFIELD = 0.1
The simulation run and it calculates the migration behavior.
However, for charged system I use this INCAR:
NELECT = (total number of electrons) - 1
LDIPOL = .TRUE.
IDIPOL = 3
DIPOL = 0.5 0.5 0.5
EFIELD = 0.1
when I include the charge into the supercell system, the simulation did not progress and stops immediately, saying "Warning: For charged systems, the potential correction is currently only implemented for cubic supercells. VASP will stop if the supercell is not cubic and LDIPOL is used." Then, I turned off the LDIPOL and run the simulation. The INCAR tags I used was:
NELECT = (total number of electrons) - 1
LDIPOL = .FALSE.
#IDIPOL = 3
#DIPOL = 0.5 0.5 0.5
EFIELD = 0.1
Now I have few questions regarding the methodology to calculate the migration behavior of ions in presence of external electric field.
1. When we calculate a migration barrier, are we calculating the migration barrier of an ion, or simply of an atom (with the electronic structure relaxing self-consistently around it)?
2. Does removing (or adding) one electron from the supercell correspond to creating an ion at the site I intend, or does it just add charge to the system as a whole? What is the physical effect of adding this charge? I now doubt that adding a charge to the supercell system does not have anything to do with making Cu+ ions. Can you please help me clarify it?
I have included the INCAR files for both neutral and charged system here.