  We present a detailed model atmosphere analysis of massive white dwarfs with
$M > 0.9~M_\odot$ and $T_{\rm eff}\geq11,000$ K in the Montreal White Dwarf
Database 100 pc sample and the Pan-STARRS footprint. We obtained follow-up
optical spectroscopy of 109 objects with no previous spectral classification in
the literature. Our spectroscopic follow-up is now complete for all 204 objects
in the sample. We find 118 normal DA white dwarfs, including 45 massive DAs
near the ZZ Ceti instability strip. There are no normal massive DBs: the six
DBs in the sample are strongly magnetic and/or rapidly rotating. There are 20
massive DQ white dwarfs in our sample, and all are found in the crystallization
sequence. In addition, 66 targets are magnetic (32% of the sample). We use
magnetic white dwarf atmosphere models to constrain the field strength and
geometry using offset dipole models. We also use magnetism, kinematics, and
rotation measurements to constrain the fraction of merger remnant candidates
among this population. The merger fraction of this sample increases from 25%
for 0.9-$1~M_{\odot}$ white dwarfs to 49% for 1.2-$1.3~M_{\odot}$. However,
this fraction is as high as $78_{-7}^{+4}$% for 1.1-$1.2~M_{\odot}$ white
dwarfs. Previous works have demonstrated that 5-9% of high-mass white dwarfs
stop cooling for $\sim8$ Gyr due to the $^{22}$Ne distillation process, which
leads to an overdensity of Q-branch stars in the solar neighborhood. We
demonstrate that the over-abundance of the merger remnant candidates in our
sample is likely due to the same process.
