Hey WildIris,
Good to hear from you again and thanks for your updates 
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[quote]I return with an update—and questions, naturally! I decided to nix the coils and go all latex. My alignment looked better (though still not perfect) with only the soft talalay directly on the springs, but it just didn’t feel as luxurious as multiple layers of latex, and since a shorter mattress lets me see more of my nifty vintage headboard, I didn’t want to pile up a bunch of layers of latex on top of the springs.
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Good work on your continued DIY experiments, WildIris! To your point, an all latex bed certainly provides a luxurious feel to those who are fans of latex foam’s unique qualities, especially when the uppermost comfort later is Talalay latex. That your new lower profile layering allows you to enjoy views of your cool vintage headboard is an added bonus 
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[quote]So I folded over the firm talalay layer to make a 6" support layer and plopped the soft on top. Seemed like a good place to start. First problem: I think the firm is too firm, because a few minutes after initially lying down on my side, it feels really hard under my hip and shoulder. I’m waking up uncomfortable multiple times every night, and I think that’s why. Slight soreness in hips as well.
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Is your current soft Talalay layer the 2"/ 20 ILD one you were using before? In your original question, we were discussing the differences between adding a 1" layer to that one vs. getting a 3" layer of the same firmness. If you are using the 2" layer, you may need a thicker one to allow your shoulders and hips to sink into the comfort layer a bit more for your side sleeping profile. Now that you’re going with an all latex setup, the next question is what are you using for a foundation?
[quote]Second problem: my back still isn’t quite straight when I take pictures. Sigh. I emailed the vendor but never heard back. I’m at a bit of a loss.
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Don’t give up on your manufacturer yet, WildIris. If they have a web site, use the Contact Us page to find and use all points of communication until you hear from them. Many offer chats, as well as toll-free numbers in addition to email; keep trying and you should hear from them at some point, hopefully sooner than later. If you’re working with a TMU trusted member, we can help you reach them as well.
[quote]So here’s what I’m thinking: try a medium Dunlop layer from Sleep on Latex (stress free, thanks to their return policy) as the middle layer, and see if that softens it up enough. Just so you don’t have to look it up, that would mean ~36ILD talalay at the bottom, then ~30ILD Dunlop, then ~19ILD talalay on top. Please let me know if there’s any reason not to alternate between the two types of latex.
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Thanks for considering trusted member Sleep On Latex, they are extremely knowledgeable with the latex category and will be able to help you sort through many of your latex comfort and support questions. I like where your going with your proposed Talalay 36 ILD/ bottom and Talalay 19 ILD on top, and yes, you can absolutely alternate Talalay latex with Dunlop. You might consider asking SOL if they would recommend a different Dunlop density, such as 24-28 ILD for the transition layer, based on your 5" 10" 150lb side sleeping profile. You may find that or a similar combination would a allow proper amount of “sink” for your shoulders and hips, offering a more correct alignment of your spine.
Sleep On Latex will be able to guide you through the layering selection process as they are deeply experienced with latex component design and have many years of working directly with consumers. I think you are heading in the right direction, WildIris. Consulting directly with an experienced latex manufacturer will enable you to construct an all latex mattress that would be best suited for your individual comfort and support needs. Thanks again for dropping in with your updates and let us know what next steps you’ll be taking when you can. Also, a big Thank You goes to @Mattrebuild for his excellent contributions and his visual support aids courtesy of expert trusted member FloBeds, those were most helpful in demonstrating correct spinal alignment among various sleep surfaces and firmnesses. We appreciate and encourage knowledgeable consumers’ support of others in their quest for a restful night’s sleep .
Thanks,
Sensei