Hi Lborducci.
Welcome to our Mattress Forum! 
I would personally not have any concerns with fumes or dust mites originating from a pocket coil. As far as VOCs or odor, the non-woven fabric used to “pocket” the coils is innocuous this is not something that people complain about. While certain products may be dust mite resistant if the food source and environment are appropriate you can get dust mites in any style of mattress. The key would be keeping their main food source (skin cells) to a minimum, and keeping the environment low in humidity (Dust mite population prefers a warm, moist environment that they need to survive)
As far as pocket coils changing firmness/shape over time… all items fatigue over time and a pocket spring unit would not be an exception. Generally, the spring units would not be the weak link within the mattress and you would notice changes much more in the foams in the upper layers because of the mechanical stress they undergo and because in time, with aging, they will gradually lose resiliency and develop impressions. Innersprings tend to hold their shape well (poor quality steel would lead to changes much more than a higher quality steel). Usually firmer latex will be more durable than an innerspring but neither of them would tend to be the weak link of a mattress.
While reviews or other people’s experiences can be useful to gauge the reputation of a business, they won’t tell you much if anything about the suitability, quality, durability, or “value” of a mattress as it relates to you (see post #13 here), and I recommend to take them with a grain of salt and to avoid considering them as any sort of a reliable indicator of the appropriateness of any particular product for your needs.
CocoMat mattress components (Dunlop latex, rubberized coir, rubberized horsehair (expensive), seaweed, essential oils, cactus fiber and others) are high quality materials and would meet the quality/durability guidelines. The company focuses on natural premium materials and unusual products and designs. Depending on your personal value equation you would be looking at a much higher price point (we are talking 10K) and firmer overall feel, although they have two firmness choices for some mattresses. Some people like the feel of it while others don’t so it is of course again a matter of preference and your personal value equation. While Coco-mats are not commonly found domestically they recently opened a showroom - Coco-Mat USA in New York, and the mattresses themselves have been around for quite some time.
I hope this helps.
Phoenix