  The valley Zeeman physics of excitons in monolayer transition metal
dichalcogenides provides valuable insight into the spin and orbital degrees of
freedom inherent to these materials. Being atomically-thin materials, these
degrees of freedom can be influenced by the presence of adjacent layers, due to
proximity interactions that arise from wave function overlap across the 2D
interface. Here, we report 60 T magnetoreflection spectroscopy of the A- and B-
excitons in monolayer WS$_2$, systematically encapsulated in monolayer
graphene. While the observed variations of the valley Zeeman effect for the A-
exciton are qualitatively in accord with expectations from the bandgap
reduction and modification of the exciton binding energy due to the
graphene-induced dielectric screening, the valley Zeeman effect for the B-
exciton behaves markedly different. We investigate prototypical WS$_2$/graphene
stacks employing first-principles calculations and find that the lower
conduction band of WS$_2$ at the $K/K'$ valleys (the $CB^-$ band) is strongly
influenced by the graphene layer on the orbital level. This leads to variations
in the valley Zeeman physics of the B- exciton, consistent with the
experimental observations. Our detailed microscopic analysis reveals that the
conduction band at the $Q$ point of WS$_2$ mediates the coupling between $CB^-$
and graphene due to resonant energy conditions and strong coupling to the Dirac
cone. Our results therefore expand the consequences of proximity effects in
multilayer semiconductor stacks, showing that wave function hybridization can
be a multi-step process with different bands mediating the interlayer
interactions. Such effects can be exploited to resonantly engineer the
spin-valley degrees of freedom in van der Waals and moir\'e heterostructures.
