We apply alternative representations of Holstein-Primakoff boson expansion to the particle-rotor model as useful probes to test the stability and the physical contents of the exact solution. The diagonal representations with the total and single-particle angular momenta along the same axis (longitudinal case) and along two perpendicular axes (transverse case) are employed according to the system with rigid or hydrodynamical moments of inertia (MoI). The longitudinal case gives the normal wobbling mode for rigid MoI, but does not give a stable solution for hydrodynamical MoI. The transverse case applied for hydrodynamical MoI describes a few low spin states with reduced alignment of the total spin in the parallel direction to the single-particle spin, but these states differ from the wobbling mode which involves a quantized unit of rotational angular momentum. Employing the angular-momentum-dependent rigid MoI which is derived from the self-consistent Hartree-Fock-Bogoliubov equation together with angular momentum and particle-number constraints, we obtain good fitting not only to the energy-level scheme, but also to the electromagnetic transition rates and the mixing ratio for $^{135}\mathrm{Pr}$.