In what percent of cases showing spreading of cancer, presumably due to metastasis, is this actually the result of viruses seeding multiple locations and becoming the underlying cause of new tumors appearing?

We would say this is at least 90% of metastatic lesions being due to a re-seeding by pre-existing local, the spread from distant chronic virus infection. We have commented previously that even when there is an appearance of multiple tumors within the same organ, such as the liver, it might not be the result of spread from an initial lesion that was the first to appear but a set of separate lesions because there is widespread virus throughout the organ causing malignant transformation in a number of locations. So the problem of metastasis is a kind of red herring and is the cause of much confusion, and that is most unfortunate as the cancer problem is not being addressed adequately because of faulty understanding of its origins and the sequence of progression being all dependent on viruses in the majority of cases.