Context. Small-scale eruptive phenomena in the solar corona, including miniature flux ropes and associated cool plasma condensations, are not fully understood despite the increasing availability of high-resolution observations. In particular, phenomena such as flux rope eruptions with trapped mini-prominence and post-flare coronal rain for miniature loops have not been extensively explored within a unified magnetohdrodynamic (MHD) model. Aims. We aim to establish observational diagnostics of mini-prominence eruptions and post-flare coronal rain by bridging MHD simulations with synthetic observables across multiple wavelength regimes from the extreme ultraviolet (EUV) to optical. Methods. We performed forward modeling based on a 2.5D resistive MHD simulation that captures homologous flux rope eruptions, in situ condensation leading to a mini-prominence, and subsequent post-flare coronal rain. Synthetic diagnostics (imaging and spectra) were obtained using optically thin approximations for EUV and UV emissions, as well as a nonlocal thermodynamic equilibrium (NLTE) radiative transfer treatment for the Hα line. Instrument resolution effects were incorporated to enable direct comparison with current observational capabilities. Results. The synthetic EUV emission reveals the flux ropes as bright rim-like structures. The corresponding UV diagnostic shows bright region, which is cospatial with the dark core due to embedded cool plasma (~ tens of kK) inside the flux rope, identifying a mini-prominence carried by the erupting flux rope. Spectral synthesis of Si IV 1402.77 Å indicates an upward motion of the mini-filament and reveals the presence of two predominant velocity components during eruption. At a later stage, thermal instability in post-flare arcades produces coronal rain with temperatures of ≈104 K. The EUV diagnostics reveal brightening at the downstream of the rain blob, indicating localized heating associated with compressional effects. The Hα spectral synthesis shows enhanced absorption signatures and redshifted profiles corresponding to downflows of the coronal rain blobs up to ≈23 km s-1, whereas the Si IV 1402.77 Å spectral profile shows the maximum downflow velocity of ≈50 km s-1, highlighting the evidence of thermodynamic and kinematic structuring within the falling rain blobs. Conclusions. The synthetic diagnostics provide clear, multiwavelength signatures that can guide future high-resolution observations, highlighting the importance of small-scale reconnection-driven processes in shaping the multithermal structure (i.e., MK to kK) of the solar corona.