I stand corrected about the dual gauge and the NAs/NBs.
According to Rod Milne's book on WAGR EE locos, the three KAs served for their first 10 years or so pretty much exclusively on the Esperance and Leonora lines. One was then withdrawn from service, a second was converted back to NG (and is now preserved), and the third was transferred to Forrestfield, from where it occasionally operated stone trains to the Avon Valley until it was withdrawn. So it seems that there was really only one KA that saw much service at the western end of the SG network.
The DFZs you saw at Forrestfield may well have been in transit between Albany and Geraldton.
As far as I know, the only narrow gauge trains that operate regularly through the Avon Valley nowadays are the CBH grain trains, which, when operated by CBH class motive power and CBH wagons (as they usually are) look identical to their SG equivalents.
I don't agree with your comments about everyone having to buy seven models each.
Let's look at a hypothetical ADK/ADB in Sn31/2. These trains operated in Perth for more than 20 years on all suburban lines, and have since operated in Auckland for more than 20 years on all suburban lines. The most popular scale/gauge combination in both WA and NZ, by far, is Sn31/2. Perth has a population of nearly two million, and Auckland about 1.5 million. There's never been any eastern states H0 scale-style r-t-r narrow gauge available for either WA or NZ, in any scale. So the problem is lack of supply, not lack of demand. If someone were to produce a model of this train in Sn31/2, it would sell, probably equally as well as the eastern states outline emus and dmus that Southern Rail is already making. The potential customers for the model wouldn't be restricted to existing Sn31/2 modellers or magazine readers. Just about anyone interested in modelling WA or NZ, including those who don't already have any such models, would be a potential customer. Of course they wouldn't sell their TGVs and Mallards to model WAGR or NZR instead. They'd just run the ADKs/ADBs on the same layout alongside them. The models would also be of interest to people who otherwise wouldn't even consider buying a model train, eg the many people here in Perth who follow the highly publicised "Lost Perth" website.
You could make similar comments about the W class locomotive, which would be the logical first WAGR steam loco to produce in r-t-r Sn31/2. A total of 64 of them were built (WAGR 60, Silverton Tramway 4). The W class is one of the most preserved classes of locomotive anywhere in the world. For a start, there are preserved examples in the big railway museums in both Perth and Adelaide. Other examples are preserved in Alice Springs, Broken Hill, Collie, Hotham Valley, Peterborough and Pichi Richi. For the whole of their working lives, the W class locos were the backbone of the WAGR/Silverton fleets, respectively, and they are also the backbone of the HVR and Pichi Richi fleets. They were built in the UK by Beyer-Peacock, the records of which are preserved in a big technology museum in Manchester. A model of the W class in Sn31/2 would be of interest to people wanting to buy memorabilia or souvenirs from any of the places I have mentioned, not least because it could run on normal H0/00 track (but it wouldn't look right if you were to model it in H0 standard gauge, because the prototype is very obviously a narrow gauge loco). They would be of (possibly even greater) interest to the potential Australian customers for an ADK/ADB model. So if someone were to produce a model of the W class, it would sell, and the only technical issue would be that you'd have to make a choice as to which type of tender to model, because more than one tender type was used (plus you'd need a full length boiler top cowling if you were to produce a Silverton version).
I agree with pretty much all of your comments, and especially with your observations about the lack of supply. The people who make kits in WA are hobbyists who make their kits in small batches; there are no eastern states style operators amongst them.
I too really appreciate Southern Rail for producing a model of the L class. The deposit per loco is actually only $100, so you could pay a deposit for three of them with $300. I also think the initial run of the L class includes at least one version to appeal to pretty much everyone who wants an L class model. I've already put in my order.