Exploring nucleon drip lines and astrophysical rapid neutron capture process (r-process) paths in the nuclear landscape is extremely challenging in nuclear physics and astrophysics. While various models predict similar proton drip line, their predictions for neutron drip line and the r-process paths involving heavy neutron-rich nuclei exhibit a significant variation which hampers our accurate understanding of the r-process nucleosynthesis mechanism. Using microscopic density functional theory with a representative set of non-relativistic and relativistic interactions, we demonstrate for the first time that this variation is mainly due to the uncertainty of nuclear matter symmetry energy $E_{\rm{sym}}(ρ_{\rm{c}})$ at the subsaturation cross density $ρ_{\rm{c}}≈ 0.11$ fm$^{-3}$, which essentially reflects the symmetry energy of heavy nuclei. Using the recent accurate constraint on $E_{\rm{sym}}(ρ_{\rm{c}})$ from the binding energy difference of heavy isotope pairs, we quite precisely determine the location of the neutron drip line, the r-process paths and the number of bound nuclei in the nuclear landscape. ; Comment: 5 pages, 3 figures + 4-page Supplemental Materials. Revised version: Title changed, results of r-process paths and discussions added, supplemental materials included