I don’t have any cites, but I’ll give you my best WAG.
Point 1: Irradiation does not destroy leukocyte antigens. Proteins and carbohydrates (the antigens in question) are radioresistant. Gamma irradiation irreversibly cross-links DNA strands making replication impossible. GVHD is prevented because lymphocytes in the donor blood cannot multiply and attack the immunodeficient host.
Point 2: Irradiation doesn’t remove leukocytes from blood. Like I said in point 1, it just makes mitosis impossible. However, techniques do exist to remove leukocytes from blood. These would be applicable to situations where you are trying to prevent either:
A. A host immune response to donor leukocyte antigens. In this case the host is immunocompetent, therefore no GVHD worries.
B. Transmission of infectios agents ALWAYS associated with leukocytes in blood and never free-floating in the sera (as is the case with CMV, IIRC, but not HIV)
Point 3: Irradiation could damage viral DNA to the point that the virus couldn’t replicate, but the dose of radiation may be much higher than practical. This point I’m least sure about. However, there is a practical limit as to how high a radiation dose you can deliver to blood. While proteins and cell membranes are radioresistant relative to DNA, the resistance is not absolute, and RBCs and platelets will become dysfunctional at high doses. I think it’s likely that a higher dose will be required to kill viruses. As a general rule, simpler engineering is more robust engineering in biology and viruses are simplicity incarnate. Also, two of the viruses you mention (HIV, HCV) are RNA viruses (they carry RNA as their genetic material). I believe RNA is more radioresistant than DNA.