  The single-lined spectroscopic binary $\nu$ Octantis provided evidence of the
first conjectured circumstellar planet demanding an orbit retrograde to the
stellar orbits. The planet-like behaviour is now based on 1437 radial
velocities (RVs) acquired from 2001 to 2013. $\nu$ Oct's semimajor axis is only
2.6 AU with the candidate planet orbiting $\nu$ Oct A about midway between.
These details seriously challenge our understanding of planet formation and our
decisive modelling of orbit reconfiguration and stability scenarios. However,
all non-planetary explanations are also inconsistent with numerous qualitative
and quantitative tests including previous spectroscopic studies of bisectors
and line-depth ratios, photometry from Hipparcos and the more recent space
missions TESS and GAIA (whose increased parallax classifies $\nu$ Oct A closer
still to a subgiant ~ K1 IV). We conducted the first large survey of $\nu$ Oct
A's chromosphere: 198 Ca II H-line and 1160 H $\alpha$ indices using spectra
from a previous RV campaign (2009-2013). We also acquired 135 spectra
(2018-2020) primarily used for additional line-depth ratios, which are
extremely sensitive to the photosphere's temperature. We found no significant
RV-correlated variability. Our line-depth ratios indicate temperature
variations of only $\pm$ 4 K, as achieved previously. Our atypical Ca II
analysis models the indices in terms of S/N and includes covariance
significantly in their errors. The H $\alpha$ indices have a quasi-periodic
variability which we demonstrate is due to telluric lines. Our new evidence
provides further multiple arguments realistically only in favor of the planet.
