The weather here in Skåne (which is the most southern part of Sweden) is the same as you describe - unusually mild for this time of year. No doubt there has been all kinds of weather records set this unusual year!
I believe you did exactly the correct thing bringing these in and I'd just leave them in their pots and move them to the attic. Generally I don't think there is any advantage of letting frost kill the foliage - this can be convenient in some cases, but letting the bulb absorb the nutrition from the leaves should make for a stronger bulb in the end. We also have to look at the reasons to store a bulb without soil - this isn't natural for the bulb, but this again can be convenient for us and can be tolerated by many species. For us it can be space saving and it can be preferable to the alternative - frost or wet and cold soil in our gardens. However in a pot in a protected spot you have 100% control of the water supply so there is usually no need to remove the soil (unless of course you have a problem with rot and need to treat the bulb.)
Hopefully there will be an Hymenocallis expert coming along with more accurate advise, but I took the liberty of googling Hymenocallis littoralis and it seems like this one is evergreen, so my suggestion would be to let it grow on. Furthermore it seems to like some moisture in the soil.
Hymenocallis harrisiana is however described as deciduous, so probably should lose its leaves and I'd just let it do this in its own time and in lack of better advise I wouldn't water it during winter.
