Hello Awesome,
Welcome to the Mattress Forum! 
The better options or possibilities I’m aware of in and around the Cleveland/Akron/Canton, OH areas (subject to making sure that any mattress you are considering meets your own criteria and the quality/value guidelines here) are listed in post #2 here. I’m not aware of which stores may or may not carry latex, or have component systems where the layers may be changed out.
Most people end up using queen sheets just because of the exact situation you described for their full extra long mattresses, and full XL frames are also not as readily available.
The thickness of the mattress is more of a side-effect of the level of comfort your looking to create, but for most individuals, yes, a 9" latex mattress can be made to feel quite comfortable for a back/side sleeper of your body mass.
Sleep on Latex is a member here on the site, which means that I think highly of them and the quality of their products. If you’re interested in looking at other online companies, you could use the experience and expertise of the members listed in post #21 here who are all very experienced and knowledgeable and specialize in providing the type of help and guidance on the phone that can help you make good choices. There are a wide range of latex options included in the choices there and I believe that all of them compete well with the best in the industry in terms of their quality, value, service, and transparency.
When you can’t test a mattress in person then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are using (Talalay or Dunlop, resilience, firmness etc.) and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs, options, and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else. You can also learn about any exchange, return, or any options they have available to customize a mattress after a purchase to help lower the risk of an online purchase. These online retailers or manufacturers can also be a good “value reference” for local purchases to make sure that if you are paying a “premium” for a local purchase (in exchange for the kind of “in person” guidance, service, and value that comes with dealing with a local retailer that can help you make more “accurate” choices that you have tested in person) is not too high.
Different manufacturers will offer various cover choices to complete the component mattress (popular ones are spandex/viscose/rayon/polyester blends, or cotton/wool covers), and then over that you could place some sort of mattress protector with a waterproof membrane (polyurethane is popular). A few good references for mattress protectors are in post #89 here and this post here.
That sounds like good spacing between the slats and with good center reinforcement you should be fine. If you create your own foundation feel free to share a photo or two.
All of the latex you are likely to encounter will be what I would classify as a good quality material compared to other types of foam. If you’re interested in 100% NR, you’ll be able to find that in both Talalay and Dunlop from many of the companies I previously linked.
You’ll usually be looking at either a natural flame retardant barrier, like wool, or a non-toxic rayon fiber blended with silica to meet federal flammability standards for the items you’re considering.
The main job of the cover will be to hold the components in place and also, in some cases, to pass flammability standards. Cotton covers with wool quilted into them can be quite durable, but they will be stiffer (especially initially when new) and many of them are spot-clean only. The least obtrusive covers will tend to be the stretch-knit variety, commonly a polyester/elastin mix, or even something like a viscose/spandex/rayon blend. There are even some cotton stretch-knit covers, but you’d want to check on the thickness of those for durability. As far as “everything proof”, you’d want to rely more upon the mattress protector for that.
Since you are attracted to the idea of designing and building your own DIY mattress out of separate components that are purchased from one or several different sources, then the first place I would start is by reading option 3 in po st #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” and fun (just as you stated) where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen). If you can, I would either use the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of comfort, firmness, and PPP as a reference point or blueprint and try and “match” every layer and component in your reference mattress as closely as possible, or alternatively use a “bottom up” approach (see post #2 here).
Couch cushions aren’t my specialty, but a 40 ILD would be a pretty firm thick couch cushion. Like so many things, it would come down to your own careful testing at an upholstery shop. I have posted previously about this exact topic and there may be some information you find useful in post #4 in this thread.
Phoenix