  We present new photometric and spectroscopic observations of SN 2019yvq, a
Type Ia supernova (SN Ia) exhibiting several peculiar properties including an
excess of UV/optical flux within days of explosion, a high SiII velocity, and a
low peak luminosity. Photometry near the time of first light places new
constraints on the rapid rise of the UV/optical flux excess. A near-infrared
spectrum at $+173$ days after maximum light places strict limits on the
presence of H or He emission, effectively excluding the presence of a nearby
non-degenerate star at the time of explosion. New optical spectra, acquired at
+128 and +150 days after maximum light, confirm the presence of CaII$\lambda
7300~$\r{A} and persistent CaII NIR triplet emission as SN 2019yvq transitions
into the nebular phase. The lack of [OI]$\lambda 6300~$\r{A} emission disfavors
the violent merger of two C/O white dwarfs (WDs) but the merger of a C/O WD
with a He WD cannot be excluded. We compare our findings with several models in
the literature postulated to explain the early flux excess including
double-detonation explosions, $^{56}$Ni mixing into the outer ejecta during
ignition, and interaction with H- and He-deficient circumstellar material. Each
model may be able to explain both the early flux excess and the nebular [CaII]
emission, but none of the models can reconcile the high photospheric velocities
with the low peak luminosity without introducing new discrepancies.
