Hi 123sleep.
Welcome to our Mattress Forum. 
Thank you for providing the visuals of the “depression” which clearly shows more than normal indentation for your 9" Dunlop mattress. It is true that softer latex (which is generally less dense) would be a little less durable than a firmer latex (which will be a higher density) especially in the uppermost layers of the mattress where it undergoes the most mechanical stress, but I would keep in mind that typically all latex is most durable compared to other types of foam. An extra soft latex layer will usually show signs of deterioration faster but certainly not in the first few months or years of use and not for normal rage BMI individuals. It looks to me that one or more of the layers may be defective, which can certainly happen from time to time, and this is when a warranty comes in place.
A GOLS-certified latex will always carry a certificate whereas natural latex is not often certified. I am not clear from the seller’s reply if this mid-layer is missing the certification label … or …if the supplier themselves did not provide the actual certification. Should this layer be in fact natural latex then it won’t carry an organic certification but if the mattress was advertised as all latex within being organic, then the manufacturer/seller should have a certification to back it up otherwise it can turn into a case of misleading representation. Generally, there is nothing wrong with natural latex, but it seems that you’ve been told that all layers are organic latex so you may want to clarify it.
GOLS is a certification for organic farming methods and/or sustainable production methods, as well as avoiding the use of harmful chemicals in agricultural practices. If interested, you can read more about certifications here.
These “hard edges” are referred to as “elephant skin” and it is quite normal. This “skin” is formed at the point of contact between the latex and the sides of the mold when the latex rubber is poured and baked into the mold. Shaving/Cutting or cutting off the entire side or just the top edges of the molded Dunlop. This is a common practice to remove the “hard skin” on the sides. The yellowish color can be indicative of the slab being sourced from a different company or even a different batch from the same company.
In some unfortunate circumstances where the latex has been excessively exposed to UV or oxidation for a longer period of time, this could result in the latex actually aging prematurely and could feel a bit harder. The S marking indicates the top or the softer side of this particular layer should be placed up. Also, some of the latex producers use fillers in their specialty products (such as TGs “Talalay mineral”) but I do not think this is the case here.
It is best to test each layer on the floor to rule out the base as being the cause of any sagging, as Mattrebuild suggested. A foundation that is not under load can look at first glance straight and sturdy when inspected, but it may start sagging under the weight of the mattress and the people sleeping on it especially if it is missing the center support.
I’d do them both as they can reveal different things. I’d be curious to see some pics of how deep the weight sinks in different areas of the layer(s) with the layers on the floor. While rare, latex failure does sometimes occur. We’ve already seen visible sagging of 1.5" or more. If your diagnostic testing reveals that this is localized to one layer alone you can certainly ask for a replacement layer. Either way, depending on what you find you’d be right to call upon the warranty for a layer replacement or even both uppermost layers if they are defective. Make sure to document well your findings and contact the seller.
The impression (depression depth) that warrants a return or warranty is different for different sellers; you’d have to read their policy to determine what the limit is. FYI large percentage of warranty claims are denied because the “unweighted” impressions in the mattress aren’t deep enough to qualify for warranty coverage even though the softening of the foam has resulted in the loss of comfort and support and the mattress is no longer suitable to sleep on. Also keep in mind that warranties are not guarantees of refunds, but either a replacement or repair of your current product, at the discretion of the company.
Good luck and keep us posted.
Phoenix