John R. Brown wrote that hybrids of kumquats with PT are not viable. I think I read the same thing in Swingle's articles. This is what we observe when your Fukushi x PT seedlings die. And it is believed that in order to get a hybrid of these species, another kind of intermediary is needed. For example, an orange, as in Thomasville. However, you, orangedays, they are not all seedlings died. Perhaps the claim of non-viability concerns the more common Nagami and Meiwa kumquats, and hybrids with other varieties are able to survive?