Adding some dilute fertilizer for formosanum is good. I am thinking your pardalinum x kelloggii cross will like the cool greenhouse that stays at or above freezing, and they will still grow slowly. But
@Steve2020 will know best.
Your trumpet seedlings can also be kept growing like formosanum, but its not mandatory. Just know that if the trumpets naturally die back (if they didn't like transplanting, for instance), they will need a cold conditioning to get them to sprout again. So your options would be to (1)keep them growing all through your winter, or (2) keep them growing in the warm greenhouse for an extra 1-1.5 months and then to the cold for the rest of the winter, to come up again at your normal spring time outside.
Treat your offsets like you would the mother plant. Even frozen soil won't hurt them. The leaves might wither, but that's ok.
Usually the first cotyledon from the seed ( or the first leaf from the seed bulb[as with pard. x kelloggii]) is actually a bit more cold hardy than the leaves that come after. Even so, none of the lilies we talk about here have leaves that will be frozen off by minus 1C.
