What's the difference between a witch and a wych?
Sometimes nothing at all.
Wych at the beginning of a word, as in wych-elm and wych-hazel means having bendable branches.
(Sometimes, just to stop things being easy, this wych is spelled witch: so, witch-hazel is nothing to do with, well, witches.)
As for the rest: witchweed:
really are genuinely to do with witches and magic. Well, when I say genuinely, I'm really only talking about derivations - I know nothing of spells. Witchweed is one of various Striga weeds of grain crops in Africa and Asia; witches' butter is the fungus Exidia nigricans; and witches' broom is a ball of twigs on a tree usually caused by a Taphrina fungus.
The witch that's a fish, Glyptocephalus cyanoglossis:
might be so called because it looks rather sinister and magical, like a witch.
But witchetty grubs are something else entirely.
Nuts and Bolts: wych witch? The pliable-branches word comes from the Old English wice, probably from the Germanic wik-, which means bend. The magical-person word comes from the Old English wicca, related to the Middle Low German wicken, to conjure, and the Swedish vicka, to move to and fro. Witchetty comes from the Adynyamathanha wityu, hooked stick, and vartu, grub - so, all boring and irritating people take note, saying witchetty grub is strictly speaking an example of tautology.