How verified experimentally is the Marginal Value Theorem for viruses?
I'm not an expert but it seems purely theoretical. There are some fairly basic criticisms that can be made of this model. First it assumes that viruses tend to optimize propagation, a perfectly reasonable assumption. The clear prediction is that propagation rate should be inversely related to mortality rate. Quickly checking a list of the most common viruses shows you that this relation is not that strong, if there at all. The second issue is that R0 is a very abstract measure. Viruses do not propagate in a straightforward manner. Rabies relies on animals biting each other, HIV sexual contacts, etc. On average each virus will still have an R0, but I'm not sure it is so sensible to assume it will trade-off with mortality as it relies on the animals behavior (although with rabbis for example the animal behavior is actually modulated by the virus itself to increase propagation). Lastly epidemics usually start with a zoonotic disease randomly mutating and jumping species. They often have quite different effect on different species so here again I am not sure it is completely reasonable to assume that the mortality rate of a virus will tradeoff with infection rate.

It's still a useful tool to think about how viruses should behave. As you noted there also are criticisms about the MVT to describe animal foraging. But mathematically, under some assumptions, it is the optimal solution. So any deviation you see from that tells you your assumptions were wrong.