Hi Flana,
Almost all the “foam” you will find in any mattress is open cell polyurethane and closed cell foam isn’t typically used in mattresses. It’s really just a matter of degree and “how open” they are.
The only way to validate this would be based on the actual certifications they have or on testing results.
These are all personal choices that each person needs to make about the types of materials they are comfortable using in their mattress. I would keep in mind that “viscose” foam is memory foam.
This would depend on what they mean by “pure latex”. If they are talking about mattresses that only use latex and no other types of foam in the mattress then they certainly do exist. If they are talking about latex that is only made from rubber (either natural or synthetic) and don’t use any other substances to manufacture the latex then they are correct and there will always be a small amount of other substances used to foam and manufacture the latex (see post #8 here). The reason that people would use latex vs polyfoam is because they prefer it but if you are comfortable with polyfoam then in higher densities it can certainly be a high quality and durable material. There is more about the different types and blends of latex in post #6 here.
There is no right or wrong here and you won’t find any consensus on these types of choices … only different preferences and different opinions. What’s important is to decide which type of material you prefer regardless of what anyone else (including a mattress manufacturer) may prefer. Their preferences or opinions may be very different from your own or from other manufacturers (see post #8 here).
I would assess the quality/durability of a mattress based on the type and density of the foams inside it regardless of the manufacturer that makes the mattress or the foam (see this article).
Phoenix