Can a species be both umbrella species and keystone species?

i think yes ; mainly because human plays a major role in our ecosystem ,for example mining areas and even human settlements do change our environment from its original to something else.

yes

competative antagonistic viral (although here it is expected to be so) infectious.

Yes, there is one that i know of. The Cuckoo bird lays its eggs in the nest of a host bird of another species, usually one with similar looking eggs to its own. The host bird then looks after the egg...

This is a symbiotic relationship called mutualism.