  SN~2018aoz is a Type Ia SN with a $B$-band plateau and excess emission in the
infant-phase light curves $\lesssim$ 1 day after first light, evidencing an
over-density of surface iron-peak elements as shown in our previous study.
Here, we advance the constraints on the nature and origin of SN~2018aoz based
on its evolution until the nebular phase. Near-peak spectroscopic features show
the SN is intermediate between two subtypes of normal Type Ia: Core-Normal and
Broad-Line. The excess emission could have contributions from the radioactive
decay of surface iron-peak elements as well as ejecta interaction with either
the binary companion or a small torus of circumstellar material. Nebular-phase
limits on H$\alpha$ and He~I favour a white dwarf companion, consistent with
the small companion size constrained by the low early SN luminosity, while the
absence of [O~I] and He~I disfavours a violent merger of the progenitor. Of the
two main explosion mechanisms proposed to explain the distribution of surface
iron-peak elements in SN~2018aoz, the asymmetric Chandrasekhar-mass explosion
is less consistent with the progenitor constraints and the observed blueshifts
of nebular-phase [Fe~II] and [Ni~II]. The helium-shell double-detonation
explosion is compatible with the observed lack of C spectral features, but
current 1-D models are incompatible with the infant-phase excess emission,
$B_{\rm max}-V_{\rm max}$ color, and absence of nebular-phase [Ca~II]. Although
the explosion processes of SN~2018aoz still need to be more precisely
understood, the same processes could produce a significant fraction of Type Ia
SNe that appear normal after $\sim$ 1 day.
