Oh, boy! I look forward to running out of superlatives...as always!
Here are my two yen worth on "trusting the original." While it's probably fool-hearty to use a completely scale airfoil in a 1/6 scale P-51, I don't think this piece of common wisdom should be carried over to, say, a 1/3 scale Bleriot. On the contrary, in my opinion, the larger the model and the more the model construction techniques parallel the construction of the original (which in the case of most of our WWI models is quite considerable), the more we can trust original as a guide for the construction of the model.
So with a model like the Halberstadt, I would trust the original to a quite considerable degree. For example, I would trust that a scale airfoil with scale incidences will fly it well. In other words, I would personally be loath to redesign the original. To take just one example, if the original aircraft (such as the Sopwith Camel) had its ailerons hinged at the bottom surface of the wing, I wouldn't hinge my model's ailerons in the middle or on top just because common modeling wisdom says that bottom-hinging may induce adverse yaw.
You mention an article on biplane incidence written in 1927 (10 years after the Halberstadt was built). Presumably, this article is talking about the aerodynamics of full-scale biplane design. So following the advice of the article really has nothing to do with "translating" the original into model form. Rather you would be choosing to redesign the original in light of subsequent discoveries in the field of aeronautical engineering. That seems like a slippery slope to me.
I've read and participated in a number of "Halberstadt droop" discussions and I'm satisfied that it was purposeful (vs. those who argue that it was essentially a construction fault). As to what the purpose of the droop was, that's still a head-scratcher. My own wild guess is that the droop results from the designers desire to have a more undercambered wing combined with an understanding of the value of washout. Aerodynamically, it seems like the droop would act like a small flap.