We investigate the effect of photon-axionlike particle (ALP) oscillations in the gamma-ray spectra of the fourth most distant blazar QSO B1420+326 measured by Fermi-LAT and MAGIC around the flaring activity in January 2020. We set the 95% CL upper limit on the photon-ALP coupling constant $g_{a\gamma} < 2\times10^{-11}$ GeV$^{-1}$ for ALP masses $m_{a} \sim 10^{-10}-10^{-9}$ eV. Assuming the hadronic origin of very-high-energy photons, we also estimate the expected neutrino flux and the cumulative flux from QSO B1420+326-like FSRQs at sub-PeV energies. Furthermore, we study the implications of photon-ALP oscillations on the counterpart gamma-rays of the sub-PeV neutrinos. Finally, we investigate a viable scenario of invisible neutrino decay to ALPs on the gamma-ray spectra and diffuse $\gamma$-ray flux at sub-PeV energies. Interestingly, we find that for the choice of neutrino lifetime $\tau_{2}/m_{2} = 10^3$ s eV$^{-1}$, the $\gamma$-ray flux has a good observational sensitivity towards LHAASO-KM2A.