  We present a high resolution numerical study of the sinking and merging of
massive black holes (MBHs) with masses in the range of $10^3 - 10^7 \,
\mathrm{M}_\odot$ in multiple minor mergers of low mass dark matter halos
without and with galaxies ($4\times 10^8 \, \mathrm{M}_\odot \lesssim
\mathrm{M}_{\mathrm{halo}} \lesssim 2\times 10^{10} \, \mathrm{M}_\odot)$. The
Ketju simulation code, a combination of the Gadget tree solver with accurate
regularised integration, uses unsoftened forces between the star/dark matter
components and the MBHs for an accurate treatment of dynamical friction and
scattering of dark matter/stars by MBH binaries or multiples. Post-Newtonian
corrections up to order 3.5 for MBH interactions allow for coalescence by
gravitational wave emission and gravitational recoil kicks. Low mass MBHs
($\lesssim 10^5 \, \mathrm{M}_\odot$) hardly sink to the centre or merge.
Sinking MBHs have various complex evolution paths - binaries, triplets,
free-floating MBHs, and dynamically or recoil ejected MBHs. Collisional
interactions with dark matter alone can drive MBHs to coalescence. The highest
mass MBHs of $\gtrsim 10^6 M_\odot$ mostly sink to the centre and trigger the
scouring of dark matter and stellar cores. The scouring can transform a
centrally baryon dominated system to a dark matter dominated system. Our
idealized high-resolution study highlights the difficulty to bring in and keep
low mass MBHs in the centres of low mass halos/galaxies - a remaining challenge
for merger assisted MBH seed growth mechanisms.
