Hi chs4000,
Questions like these always involve tradeoffs and where inside a range of risk you are most comfortable. There are no black and white answers in other words.
Latex can oxidize with exposure to air/oxygen/ozone, and ultraviolet light along with some of the other substances that are listed here. You can also get some sense of the different degrees of oxidation as a result of using different types of covers in post #3 here and post #3 here.
Your setup would certainly be better than sleeping directly on latex or using latex with only a single fitted sheet on top of it but it wouldn’t be as effective as using a dedicated cover around the latex in addition to fitted sheets and even here the type and thickness of the cover would make a difference … but can also affect the feel of the latex more.
Having multiple layers of sheets over the mattress may also affect the feel and performance of the latex (depending on the fabric, how tight it is over the latex, and on how stretchy it is).
Your suggested layers would certainly help slow down the oxidation or breakdown of the latex compared to other options that would be less protective but it also wouldn’t be as protective as other options or cover materials that would surround and protect the latex more effectively than what you are suggesting.
I’m also not quite clear on the purpose of the fitted sheet over the mattress and under the latex because it seems that it wouldn’t really add anything to how well the latex was protected.
While none of this would likely have a significant practical effect in short to medium term use (outside of direct exposure to substances that are very harmful in the short term) … the oxidation of latex or other “gradual” effects that happen over time and continuous exposure to substances that are harmful to latex can certainly affect the useful life of the latex in the longer term.
Phoenix