Reflections of a stormy petrel

Two more points. First, the "top-down" and "bottom-up" perspectives you describe in northern Europe were hardly mutually exclusive. You have to remember the dimension of the societies in question. You are not talking about a large state such as the Byzantine empire, not even about a confederacy of vast provincial landholders such as underlay the Frankish states. In England in the seventh century, in Scandinavia in the tenth, a king was the local boss, and he and his comitatus pretty much exhausted every man of importance in the kingdom - and most kingdoms were barely as large as a modern county, and not as populous. Once the royal family has been converted, the comitatus will become converted out of personal loyalty and individual influence; after all, they had been as much responsible for choosing the king as the king for choosing them, and their loyalty and affection was mutual. (Remember the speeches after the Ealdorman's fall in The battle of Maldon, written almost certainly by an eyewitness: the personal emotion, the direct and uncomplicated love for one's lord, can be felt.) And once these men had converted, pretty much all of society had; everyone was bound to them as they were bound to the king, by economic and personal links that strongly encouraged them to agree.

And the fact that the Landnamabok takes the trouble of mentioning Helgi the Lean's peculiar double religion hardly proves that it was common. One of the rules of textual interpretation is that common things are not mentioned, and uncommon things are. Why is that the practice of this one settler, out of dozens, is mentioned? Probably because it was unique, and remembered as such.