  We report on a study on a representative set of Fe$_{2}$P-based MnFePSi
samples by means of $^{55}$Mn NMR in both zero and applied magnetic field. The
first-order nature of the magnetic transition is demonstrated by truncated
order parameter curves with a large value of the local ordered moment at the
Curie point, even at compositions where the transition appears second order
from magnetic measurements. No weak ferromagnetic order could be detected at
Si-poor compositions showing the kinetic arrest phenomenon, but rather the
phase separation of fully ferromagnetic domains from volume fractions where Mn
spins are fluctuating. The more pronounced decrease of the ordered moment at
the $3f$ sites on approaching $T_C$, characteristic of the mixed magnetism of
these materials, is demonstrated to be driven by a vanishing spin density
rather than enhanced spin fluctuations at the $3f$ site. An upper limit of
0.03~$\mu_B$ is set for the fluctuating Mn moment at the $3f$ site by the
direct detection of a $^{55}$Mn NMR resonance peak in the Mn-rich samples above
$T_C$, showing nearly temperature-independent frequency shifts. A sharper
secondary peak observed at the same compositions reveals however the
disproportionation of a significant $3f$-Mn fraction with negligible hyperfine
couplings, which retains its diamagnetic character across the transition, down
to the lowest available temperatures. Such a diamagnetic fraction qualitatively
accounts for the reduced average $3f$ moment previously reported at large Mn
concentrations.
