Hi passionate4nature,
Many of the better sources I’m aware of for latex toppers or mattress components are listed in post #4 here. There are a couple listed there that you didn’t include but I don’t specifically keep any records of retailers who sell seconds or exchanged layers.
I don’t know if I can answer this without knowing the specifics of the layer and the design of the mattress you are planning to use it in. If you are comfortable buying “seconds” then I would use it in the same way I would use a new layer.
Dunlop has a higher compression modulus than Talalay latex (it gets firmer faster with deeper compression than Talalay once you are past the 25% compression where ILD is measured) so in an equivalent ILD it will usually feel firmer than Talalay yes. This is one of the reasons why some people like to use it in support layers but which may be “better” for a particular person depend entirely on the design goal of the mattress and the preferences of the person. Dunlop and Talalay have a different “feel” as well (see post #7 here).
Separate layers act a little bit more independently from each other and can “act” a little bit softer than a single layer but this would be more noticeable in the upper layers (say 3 x 1" layers vs a single 3" layer for example) than in the deeper layers for most (but not all) people assuming all the layers you are comparing are the same ILD.
The components/topper post I listed earlier also has some of the better sources for covers I’m aware of.
Phoenix