Romani, I think you're trying to sort out something that doesn't have have crisp definitions. Shellac, varnish and lacquer are three similar products. I've done some cabinet and furniture woodworking projects, and here's the way they've been differentiated to me:
Shellac is a dried resin dissolved in grain alchohol and there's a bit to know to use it properly. Here's an excellent article about it:
http://antiquerestorers.com/Articles/jeff/shellac.htm
As Dan points out, shellac is used in woodworking as a sealer before applying the finish coats. I often used it to seal exposed end grain so that the top coat would apply evenly over the face and end grain. It seals the grain and raises it. You give it a light sanding with fine steel wool to knock down the raised grain, then you can apply either lacquer or varnish over it. But a shellac finish does not require a lacquer or varnish top coat; it's beautiful and fairly durable all on it's own. In fact, I was surprised when I first inspected my house before buying it to find that the kitchen cabinets are all white oak with a shellac finish -- no top coat. They're kept clean and polished with an oil rather than a wax and they glow with a warm amber color. You could use shellac followed by varnish to seal and coat a wooden airplane, but you could also use just shellac. If it's good quality, it will hold up for quite awhile but eventually will deteriorate with age and weather exposure.
Varnish and lacquer are more like each other than either is similar to shellac. They're both made from a resin, a carrier and a dryer or accelerant. Lacquer can be clear or it can be pigmented to a solid opaque color. Varnish is either clear or tinted to a color; it's always transparent to some degree, showing the wood grain beneath. A lacquer finish is generally applied to furniture or low-wear interior trim where a high quality finish is needed. Varnish is used in high-wear, weather exposed areas like floors, baseboards, closet fittings and marine decks, where durability is more important than high finish. Varnish tends to be a heavier finish than lacquer and fills-in both grain and fine detail.
I think that Germany had everything it needed to produce either lacquer or varnish. The resin could come from indigenous trees. The German chemical industry could produce the carriers and dryers. Shellac would be limited by the amount of resin could be imported from India or Asia.
All three finishes can be either brushed or sprayed on. Spraying equipment was available and used in WWI Germany.
I don't know which of the three was used in Germany on their wooden airframes. There's no reliable way to tell from b&w photos; you'd have to look at what Idfleig ordered and -- really -- what was actually available and delivered to the factories. There are Idfleig airframe finishing instructions that actually call out a particular manufacturer's varnish (yes, "varnish" IIRC), but a better indicator would be Bill of Ladings from the factories, if any survive in their archives.
Any one of the three finishes would have been adequate for short term sealing against dirt and moisture, but if I rank them from least durable (over time) to most durable, I'd say shellac-lacquer-varnish. The wood used was very thin birch plywood, 1.5 to about 3 mm thick. it doesn't take much to seal and coat something like that. If you are being hasty, one coat of good varnish on both sides would fill and seal adequately, but it would probably look "spotty" with dull and glossy areas. Close-up pictures of Albatros D.II and D.III natural wood finish fuselages show a very even, glossy top coat. I'd guess that the norm was at least two coats, one to seal and one to finish, to get that kind of appearance.
Just my own opinion, with no documentary evidence to support it: at least the Albatros top coat looks like a varnish finish to me -- not a lacquer or a shellac. Shellac and lacquer both go on thinly and would require several coats and a lot of hand sanding and rubbing to get that kind of finish. You could get the same quality finish with a couple of coats of good varnish and it would hold up better to outside exposure than shellac or lacquer. Perhaps, as Dan suggested, they did use a first coat of shellac to seal, followed by a top coat of varnish for weather protection. That would certainly account for the "warm, straw-colored" finish that's so often described.