  High-resolution observations of several debris disks reveal structures such
as gaps and spirals, suggestive of gravitational perturbations induced by
underlying planets. Most existing studies of planet--debris disk interactions
ignore the gravity of the disk, treating it as a reservoir of massless
planetesimals. In this paper, we continue our investigation into the long-term
interaction between a single eccentric planet and an external, massive debris
disk. Building upon our previous work, here we consider not only the
axisymmetric component of the disk's gravitational potential, but also the
non-axisymmetric torque that the disk exerts on the planet (ignoring for now
only the non-axisymmetric component of the disk self-gravity). To this goal, we
develop and test a semi-analytic `$N$-ring' framework that is based on a
generalized (softened) version of the classical Laplace--Lagrange secular
theory. Using this tool, we demonstrate that even when the disk is less massive
than the planet, not only can a secular resonance be established within the
disk that leads to the formation of a wide gap, but that the very same
resonance also damps the planetary eccentricity $e_p$ via a process known as
resonant friction. The resulting gap is initially non-axisymmetric (akin to
those observed in HD 92945 and HD 206893), but evolves to become more
axisymmetric (similar to that in HD 107146) as $e_p(t)\rightarrow0$ with time.
We also develop analytic understanding of these findings, finding good
quantitative agreement with the outcomes of the $N$-ring calculations. Our
results may be used to infer both the dynamical masses of (gapped) debris disks
and the dynamical history of the planets interior to them, as we exemplify for
HD 206893.
