I have enjoyed reading attempts at reconstructing the early elements of the Balder myth in Anatoly Liberman. 2004. Some Controversial Aspects of the Myth of Baldr. Alvíssmál 11, 17–54.

"Yet it cannot be doubted that in the ancient myth the plant which killed Baldr looked harmless. At an early stage in the development of Baldr mythology, a certain plant, for instance, the reed, was probably sacred to Baldr, as the myrtle was sacred to Aphrodite."

"Whatever the age of the motif of all things swearing not to hurt Baldr, this plant was passed over as loyal to Baldr by definition. As usual, a god or a hero endowed with supernatural strength can be killed only with his own sword. In Baldr’s case it was his special plant that had the power to kill him."

"Although we can reconstruct, from the dubious debris, his previous domains, sunlight and vegetation being among them, we do it no better than an etymologist who reconstructs the old meanings of a familiar word."

"But its message was clear: an unforgivable crime leads to the collapse of stability, and Óðinn heard this message as well as did Snorri, who happened to live at a time when law and order broke down and the societal chaos was too deep even for Þórr. Once Baldr fell, the gods did not succeed in any of their ventures. They could not launch the ship Hringhorni; even Hyrrokkin’s “horse,” a creature akin to Fenrir, overpowered Óðinn’s berserks; Hermóðr’s mission failed; Baldr remained with Hel, and, emboldened by the gods’ impotence, the giants, previously kept at bay, attacked and destroyed Ásgarðr. In the past, the gods’ defeats had always been temporary. Baldr’s death was not the first in the eddic universe, but it was the first irreparable loss."