We investigated the interplay between the thermal decay of the low temperature metastable HS state and the thermal hysteresis of the metal diluted spin-crossover complexes, [Fe_xMn_1-x(dpp)₂(NCS)₂].py (dpp = dipyrido[3,2-a:2′,3′-c]phenazine and py = pyridine). The thermal decay temperature T(LIESST) could be shifted into the quasi-static hysteresis range by several means: increase of the Mn content, increase of the temperature scan rate of the heating experiment, and irradiation using visible light. A detailed investigation of the isothermal relaxation of the metastable HS state at various temperatures was made and compared to simulations. The theoretical investigation based on the usual macroscopic master equation was focused on the regime change generated by the meeting of T(LIESST) and the thermal hysteresis range, and provided a qualitative agreement with the experimental data. The search of quantitative agreement led to introduction of parameter distributions and allowed illustration of the already-known drawbacks of the mean-field Ising-like model. (Graph Presented).