  Understanding the differences between the distribution of quarks bound in
protons and neutrons is key for constraining the mechanisms of SU(6)
spin-flavor symmetry breaking in Quantum Chromodynamics (QCD). While vast
amounts of proton structure measurements were done, data on the structure of
the neutron is much more spars as experiments typically extract the structure
of neutrons from measurements of light atomic nuclei using model-dependent
corrections for nuclear effects. Recently the MARATHON collaboration performed
such an extraction by measuring inclusive deep-inelastic electron-scattering on
helium-3 and tritium mirror nuclei where nuclear effects are expected to be
similar and thus be suppressed in the helium-3 to tritium ratio. Here we
evaluate the model dependence of this extraction by examining a wide range of
models including the effect of using instant-form and light-cone nuclear wave
functions and several different parameterizations of nucleon modification
effects, including those with and without isospin dependence. We find that,
while the data cannot differentiate among the different models of nuclear
structure and nucleon modification, they consistently prefer a
neutron-to-proton structure function ratio of at $x_B \rightarrow 1$ of $\sim
0.4$ with a typical uncertainty ($1\sigma$) of $\sim0.05$ and $\sim0.10$ for
isospin-independent and isospin-dependent modification models, respectively.
While strongly favoring SU(6) symmetry breaking models based on perturbative
QCD and the Schwinger-Dyson equation calculation, the MARATHON data do not
completely rule out the scalar di-quark models if an isospin-dependent
modification exist.
