So what’s the “hæleð” in hæleð-helm ‘helmet of invisibility?
In modern Danish the noun hæler is a person who hides and distributes stolen goods. But not being the actual thief him-/herself but more in the sense of a distributor.
It is familiar with an older verb hæle (old norse; hæli hiding-place or getaway) meaning to hide or place someone or something out of range.
So “helmet of invisibility” could make sense.