You make a good point about Dru insanity but I do wonder if her psychic abilities would enable her to see that Dawn has been inserted into the universe rather than was originally a part of it.Just another reason why Sprusilla should've stayed together so that Dru could appear in Season 5 and come into contact with Dawn.
There is a difference between Joyce's insanity (and those that are victims of Glory) and Drusilla's insanity. Joyce's and Glory's victims' insanity is medical/ the result of the brain being physically messed with via tumour and brain sucking. Drusilla's insanity is trauma induced and her mind broke away from reality in order to cope with the psychological torture Angelus was inflicting on her.
Whilst it's extremely likely, near certain, that Dru would clock Dawn because we were told crazy people can see Dawn's true nature as the key, could it also be they type of insanity that allows someone to see Dawn's keyness? The insanity portrayed in Season 5 with Joyce and Glory's victims is not the same insanity that was portrayed with Drusilla, in which case there is the slight chance that Dru wouldn't be able to see that Dawn is the key because it's not the "right" insanity to be able to do so.