Maybe because they don't really know which cult the temple belonged to. There were many smaller cults in those days like Mithraists for example. Could even have been a Christian cult centered around some martyr or saint.
Sure why not, they were in the british isles in the 5th century. I only used it as an example because I'm reading a book about them atm. ;)
Mithraism was present in England only because Roman soldiers from else in the Empire were stationed there (the Roman army was the major factor in the spread of Mithraism). However, no Roman soldiers ever set foot in Norway, so no, it is not a reasonable hypothesis that Mithraism existed in Norway in the first millennium.