||The chromosphere of the umbra of sunspots presents highly dynamic small-scale features that, until now, were poorly understood and studied separately. These include short-dynamic fibrils, spikes, umbral microjets, and small-scale umbral brightenings. We studied the time and spectral evolution of these features and, separately, built semi-empirical models using the automated non-LTE inversion code NICOLE on Ca II 8542 SST observations. We find a remarkable agreement of results between both approaches and show that most small-scale features are counterparts of the same phenomenon. Most strikingly, the bright small-scale features are generated at the footpoints of the dark fibrils on their descent phase. The atmospheric inversions consistently capture both as a single narrow downflowing body, with a continuous variation in MHD properties, and against a gently upflowing or at rest background. The full co-evolution of these features, and their phase relationship with umbral flashes, allow us to propose a general model for the topology of the evolving chromospheric umbra. With the benefit of these complete MHD models we synthesise multiple IRIS observables using RH, with special interest on the small-scale umbral brightenings. We discuss what to expect in different model scenarios that affect the transition region above sunspots and how these can be constrained by IRIS.