I hope I don't sound tedious on this point
Hey, as they say, "the Devil's in the details." All good points.
I agree there's a big difference between innate faculties and innate concepts. We are not born with innate concepts of house, human, horse, justice or οίκος, άνθρωπος, 'ιππος, δίκαιος and then map reality to those innate concepts. Research doesn't support that. Epicurus doesn't seem to me to support that.
I will say language acquisition in children is miraculous to behold! I can see how some may have come up with a theory that we "re-discover" language because it is so magical to behold. It may be interesting to note that research has discovered that babies naturally produce every phoneme that human language uses and those that are not phonemically significant for their parents first language will be weeded out. We gradually learn to see that animal as a horse and not a 'ιππος or Pferd depending if our parents are English or American or Greek or German.
We also don't have some kind of innate Platonic Horse Form against which we compare our sensory input. It's simply the baby's constant reinforcement of "That's a horsie." Points. "Orsy!" "No, that's a cat." "Orsy!" "No, that is a dog." "Horsy!" "Right, that's a horsy! Oh, pretty horsy." I find it hard to think that Epicurus endorsed an innate horse-template to "measure" our sensory input against. But, you're right, I think I've read that sort of thing as some saying that's what prolepseis are.
I need to go back and read Laertius description of the Canon and Sedley's paper on On Nature Book 28 on language.
As for the "fourth leg," my jury is still out that there's any 4th leg at all. I'm still not entirely convinced that Laertius's "Epicureans" weren't qualifying the prolepseis or expanding the explanation of prolepseis. I also need to dig back into DeWitt's paper.
The translation is:
"Now in The Canon Epicurus affirms that our sensations and preconceptions and our feelings are the standards of truth ; the Epicureans generally make perceptions of mental presentations44 to be also standards."
Note 44 in Perseus read: Such mental pictures are caused by atoms too fine to affect sense : cf.§ 64infra; Lucr. ii. 740 sqq., iv. 722 sqq. ; Cic. N.D. i. 54. On the whole subject consult Usener's Epicurea, Fr. 242-265, and, more especially, Sext. Emp. Adv. math. vii. 203-216.
Usener Fragments 242-265 are available on Attalus's site: http://www.attalus.org/translate/epicurus2.html#us2
Especially pertinent here seems to be 255-259.
This isn't a simple topic by any means, but it is an important one. I'm enjoying the digging in!