Hi sweet,
Sorry again for all the hassles that you’ve had to go through to find a suitable mattress. That’s one of the reasons we went with - and really enjoy - our Nest Bedding Hybrid Natural Latex mattress so much is that there hasn’t been any soreness in our shoulders or hips like other mattresses. We do think the stretch cover/EcoFlex foam is part of that.
As you’ve encountered, even when you get the latex ILD right, as soon as you put on a traditional wool/cotton cover, the mattress gets too firm. That was our experience. That’s part of the reason why Nest Bedding went with this design - to make it more comfortable and contouring. It’s been a night-and-day experience between our previous wool/cotton cover mattress and this stretch-cover/EcoFlex version.
And like you, we were a little hesitant about the EcoFlex foam. But again, it’s backed by Nest Bedding’s warranty policy, and we haven’t encountered any issues with it so far. And to be honest, most of the mattresses that we had over the last two decades were the polyfoam/pillowtop versions anyway, and those lasted us for a good 6-8 years. They were also comfortable right from the start, and this Nest Bedding Hybrid Natural Latex is closest one we’ve found that matches that comfortable feel. And having experienced a number of other manufacturers, we’re pleased with the quality of Nest Bedding’s products.
The EcoFlex foam is woven into the cover, so that portion cannot be replaced from what we can tell. Only the latex foam piece may be swapped out once the cover is unzipped. And for us, we haven’t needed to worry about the latex foam. The Dunlop works fine for us. The softness of the EcoFlex stretch cover, combined with the support of the Dunlop, is a winning combo in our opinion. We get contouring comfort where we need it (shoulders/hips), and the Dunlop latex provides nice support on our backs - without causing any pain when we lay on our sides. There’s plenty of point elasticity with the cover - unlike anything else we’ve tried. We wouldn’t go back to the traditional wool/cotton covers at this point, as those were too firm for us.
Also, my wife has fibromyalgia and a strong sensitivity to smell. She hasn’t been bothered on either front with the Nest Bedding Hybrid Natural Latex mattress. No more pain like we both experienced with every other mattress, and there hasn’t been any off-gassing with our mattresses. The “Natural” in the Nest Bedding Hybrid Natural Latex title is legit. There’s no chemical smell with the EcoFlex foam, so it’s not some marketing gimmick.
As for our weights, we’re both on the lightweight side - with my wife being around 130lbs. We’re very happy with our Medium version. If you only sleep on your side, you could go with Plush. However, as the mattress softens up, you might wish you had the Medium version. We tried out both, and we both preferred the Medium. There’s quite a bit of give with the Medium version. The Plush might work right out of the gate, but could feel too soft after a while. The Medium, while a touch firm to start with, does loosen up nicely in about a week or two. And since the EcoFlex foam is going to loosen up somewhat over time, too, you might not want to go too soft with this mattress.
We came across this review for the Nest Bedding Hybrid Natural Latex mattress which very much matches our experience (this site also confirmed our too-firm experience with the Avocado mattress): Nest Owl Mattress Review 2022 (+Video) - Hand Test . As you can see, the Medium and the Plush are rather close in feel. They recommended the Medium for most sleepers, and that’s been our experience as well. If you only sleep on your side and you need a very soft mattress, the Plush could work. But if you’re a combo sleeper, the Medium might be preferable. You’ll sink in quite a bit with the Plush - whereas the Medium felt like just enough for us.
You could always try the Medium - and if it still feels too firm after a couple of weeks, you could swap out the Dunlop latex for the Plush version. We’d recommend giving the Medium a good couple/few weeks to break in, though, if you go that route.
Again, we have no regrets with our Nest Bedding Hybrid Natural Latex mattresses. Nest has done their homework and created a really nice, well-built, comfortable-yet-supportive product. We don’t give recommendations lightly. And before this particular mattress, there wasn’t a mattress out there that we’d tried or looked at that we could recommend. With this one, though, we have no hesitation recommending it. Like we said, our only “complaint” with it is that we don’t want to get out of bed in the morning, given that it’s so comfortable! 
Hope that helps, and happy to answer any other questions you may have…