Hi cachexian,
Yes … they are now in Sri Lanka. The rationale was that being closer to the source of raw materials would be a significant cost savings.
Most of the latex zoning … particularly with Dunlop … is not much more than the natural variation in the layer anyway. I don’t think it would be harmful because of the fairly small zoning variance (although some types of zoning can be counterproductive for some people) but I wouldn’t pay extra for that many zones. Your own personal testing will tell you if it is OK more than anything else but I would guess in most cases it would be fine.
I think that having a zip cover is a bonus because you can make adjustments to the mattress if you make the wrong firmness choices, change out the layers down the road if your needs or preferences change, replace just one layer of the mattress if it wears out faster than the others, and you can wash the cover from time to time. All of these I would consider to be a benefit, especially the ability to exchange a layer. If you are ordering online it can be a real benefit but if you have tested a mattress locally then a tape edged mattress which you can’t change wouldn’t be as big an issue (many local manufacturers will open it up and change a layer if necessary) although it still wouldn’t have the flexibility of a zip cover.
It could help the cover maintain its shape a little better and keep it from shifting and developing creases or wrinkles but you would also lose the flexibility of having a zip cover. Creases and wrinkles are generally not as big an issue with quilted covers as they are with thinner stretch covers though.
Yes … if you flip it regularly it will last longer because each side is subject to the stresses of sleeping on it for only half the time. while latex is the longest lasting of the foam materials and while there is some design flexibility lost with a double sided mattress in terms of layering (you shouldn’t have too much soft foam on the bottom and you can’t use progressively firmer layers in the same way) it has a durability benefit even with latex.
The latex won’t develop a crack in the middle no and with a tight cover latex stays in place very well (it’s quite “sticky” and doesn’t slide over other layers). As long as there is a layer such as quilting or a thin layer of latex that will even out the feel of the split then it will be fine and it’s a very worthwhile option for many couples. You can see more in post #2 here.
I don’t believe it’s really necessary with a latex mattress that has a zip cover no. If something for whatever reason (say moving the mattress on its side) caused the layers to shift then it’s an easy matter to open the cover and shift them back. Gluing generally isn’t necessary and you would lose the advantage of being able to exchange or change layers down the road.
Just in case you haven’t seen these … post #2 here has a link to all the Australia threads on the forum and one in another forum as well. I also amalgamated all the links to all the manufacturers and retailers that were mentioned in all the Australia threads (including the one in the other forum) in post #7 here although you already seem to have many options.
Hope this helps.
Phoenix