Prayers for you DJ. My beloved passed from melanoma five years ago. She had access to Yervoy but Keytruda was a year in the future. It did not help that her melanoma was occult, no visible lesion, sore, or wart. They never found the site of origin. Again prayers up that all will be well for you and your family.
Sorry for your loss. Unlike squamous cell and basal cell there isn’t a strong correlation between sun exposure and melanoma. Melanoma can appear on skin that barely even sees the sun or as in your wife’s case, may not even be on the visible surface. My guess as to why your wife didn’t respond to checkpoint inhibitor therapy is since it wasn’t a solar induced melanoma it had low ‘mutational burden’. Checkpoint drugs tend to do well only when there are a lot of mutations in the cancer cells that make them clearly visible to the immune system. Same reason why heavy smokers who get lung cancer do better than non smokers getting lung cancer.