The relative strength of different proximity spin-orbit couplings in graphene on transition metal dichalcogenides (TMDCs) can be tuned via the metal composition in the TMDC layer. While Formula Presented has a normal gap, proximity to Formula Presented instead leads to valley-Zeeman-driven inverted bands. Although the Formula Presented index vanishes, these systems enable a concentration-dependent topological crossover with band gap closure when graphene is stacked on a composite or alloyed TMDC layer. This is due to a nonzero Berry curvature at the individual valleys, i.e., a hallmark of the quantum valley Hall effect and a change in the valley Chern index at a critical composition ratio. Therefore, inherently, we also expect that stacked heterostructures of graphene on composite TMDC layers should host localized boundary modes due to the presence of Formula Presented- and Formula Presented-like domains with opposite valley Chern indices. In this study, we show that a Gr/(Mo-Formula Presented heterostructure with a lateral interface in the TMDC layer can indeed host valley-dependent topologically protected in-gap propagating modes, similar to those at the border of commensurate AB and BA domains in biased minimally-twisted bilayer graphene. However, the stability of these modes depends crucially on the system size. We demonstrate that the electronic behavior of Gr/(Mo-Formula Presented heterostructures evolves from a homogeneous effective medium to a superposition of domain-localized bands and zero-energy branch crossings as the domain size in the alloyed TMDC layer is increased.