In this study, we explore the interplay between topological defects and resonant phenomena in photonic crystal slabs, focusing on quasi-flatband resonances and bound states in the continuum (BICs). We identify anisotropic quasi-flatband resonances and isotropic quasi-flatband symmetry-protected BICs that exist in coupled topological defects characterized by nontrivial 2D Zak phases, originating from monopole, dipole, and quadrupole corner modes within second-order topological insulator systems. These topological defect modes, whose band structures are described using a tight-binding model, exhibit distinctive radiative behavior due to their symmetry and multipolar character. Through far-field excitation analysis, we demonstrate the robustness and accessibility of these modes in terms of angular and spectral stability. Furthermore, we investigate potential applications of the quasi-flatband resonances in light-matter interactions, including optical forces, second-harmonic generation, and strong coupling, which exhibit robust performances under varying illumination angles. These findings offer new opportunities for precise control over light-matter interactions.