Whoops, I'd always read the spell as putting the caster into the transcription process after full comprehension. I see now it's pretty clear that it still considers the knowledge given to be "another source" from your personalized sources (book, familiar).
I think the best interpretation is that for both wizards and witches, the "other source" is the caster's knowledge, and it is copied as if from another "personalized source". A wizard "reads" the knowledge like another spellbook, whereas the witch's familiar communes with the witch to access the knowledge.
Why? (Includes critiques of your suggestion in no particular order.)
* I see no reason for the process to be more complicated for a witch. Generally, the requirements to "learn" a spell are similar for a witch and a wizard, with the cost of transcription being the major difference that I see. (And it being always present, it is presumably factored in elsewhere?)
* If the scroll method was the required method for a witch, it would not be necessary to teach the familiar within the 24 hours as the wording implies. As the ONLY option would be to scribe a scroll, after which teaching your familiar is completely at your leisure, the text would not benefit from mentioning the familiar except in this context.
* The witch and familiar already share a psychic bond. I cannot think of a reason that this psychic bond could not be analogous or even superior to familiar-to-familiar communion.
* Familiar text wording frequently refers to "adding" spells to a witch's familiar. While wording also frequently refers to "teaching" or a familiar "learning", I believe that in truth a familiar is (at least when it comes to spells) functionally intended to be a vessel, like a spellbook, rather than a sentient entity which understands and safeguards comprehension for you. It stores the spells but you understand them. I believe therein lies indication that a familiar simply learning the spell once would be insufficient.
* The range of the spell is touch. In order to share a touch spell with a familiar, you must have touched yourself, whereas this spell's target is the corpse (or the blood).
* Even If the spell is shared with the familiar it would, if nothing else, need to commune with itself to store the spell. Otherwise the balance would be against wizards, and the extra complication of transcription would only be upon them.