  We study the orbital decay of a pair of massive black holes (BHs) with masses
5 * 10^5 and 10^7 M_sun, using hydrodynamical simulations of circumnuclear
disks (CNDs) with the alternating presence of sub-grid physics such as
radiative cooling, star formation, supernova feedback, BH accretion and
feedback. In the absence of such processes, the orbit of the secondary BH
decays over timescales of ~10 Myr to the center of the CND, where the primary
BH resides. When strong dissipation operates in CNDs, fragmentation into
massive objects the size of giant molecular clouds and with densities in the
range 10^4 - 10^7 amu / cm^3 occurs, causing stochastic torques and hits that
can eject the secondary BH from the midplane. Outside the plane, the
low-density medium provides only weak drag, and the BH return is governed by
inefficient dynamical friction. In rare cases, clump-BH interactions can lead
to a faster decay. Feedback processes lead to outflows, but do not change
significantly the overall density of the CND midplane. However, with a
spherically distributed BH feedback a hot bubble is generated behind the
secondary, which almost shuts off dynamical friction, a phenomenon we dub "wake
evacuation", leading to delays in the decay of possibly ~0.3 Gyr. We discuss
the non-trivial implications on the discovery space of the eLISA telescope. Our
results suggest the largest uncertainty in predicting BH merger rates lies in
the potentially wide variety of galaxy host systems, with different degrees of
gas dissipation and heating, yielding decay timescales from ~10 to ~300 Myr.
