Black Hole Mass Scaling Relations for Spiral Galaxies. II. M_(rm BH)-M_(rm *,tot) and M_(rm BH)-M_(rm *,disk)

Black hole mass ($M_{BH}$) scaling relations are typically derived using the properties of a galaxy's bulge and samples dominated by (high-mass) early-type galaxies. Studying late-type galaxies should provide greater insight into the mutual growth of black holes and galaxies in more gas-rich environments. We have used 40 spiral galaxies to establish how $M_{BH}$ scales with both the total stellar mass ($M_{*,tot}$) and the disk's stellar mass, having measured the spheroid (bulge) stellar mass ($M_{*,sph}$) and presented the $M_{BH}$-$M_{*,sph}$ relation in Paper I. The relation involving $M_{*,tot}$ may be beneficial for estimating $M_{BH}$ either from pipeline data or at higher redshift, conditions that are not ideal for the accurate isolation of the bulge. A symmetric Bayesian analysis finds $\log\left(M_{BH}/M_{\odot}\right)=\left(3.05_{-0.49}^{+0.57}\right)\log\left\{M_{*,tot}/[\upsilon(6.37\times10^{10}\,M_{\odot})]\right\}+(7.25_{-0.14}^{+0.13})$. The scatter from the regression of $M_{BH}$ on $M_{*,tot}$ is 0.66 dex; compare 0.56 dex for $M_{BH}$ on $M_{*,sph}$ and $0.57$ dex for $M_{BH}$ on $\sigma_*$. The slope is $>2$ times that obtained using core-S\'ersic early-type galaxies, echoing a similar result involving $M_{*,sph}$, and supporting a varied growth mechanism among different morphological types. This steeper relation has consequences for galaxy/black hole formation theories, simulations, and predicting black hole masses. We caution that (i) an $M_{BH}$-$M_{*,tot}$ relation built from a mixture of early- and late-type galaxies will find an arbitrary slope of approximately 1-3, with no physical meaning beyond one's sample selection, and (ii) evolutionary studies of the $M_{BH}$-$M_{*,tot}$ relation need to be mindful of the galaxy types included at each epoch. We additionally update the $M_{*,tot}$-($\textit{face-on}$ spiral arm pitch angle) relation.