Hi jdog,
OK … one at a time 
There are three local factory direct outlets near you which may be worth including in your research. If they have similar value then it’s always more accurate to actually lay on a mattress than it is to go by averages in an online purchase. They are …
http://www.midwestbedding.biz/index.html (see post #519 here)
http://www.lebeda.com/locations.html
Furniture Row® Store Locations - Store Hours & Addresses
The difference between a differential and progressive construction is really how far apart the upper layer ILD is from the layer below it. For example a 3" 19 ILD comfort layer over a 6" 36 ILD support layer would be differential. A 2" 24 ILD over a 6" 32 ILD support layer would be more progressive because the lower layer may need to “help” the thinner comfort layer with pressure relief so you wouldn’t want the difference in the ILD between the layers to be so much that you felt an obvious “firm” layer underneath it. Differential really means that the comfort layer is thick enough that it doesn’t need any “help” from the layers below it so they can be much firmer without feeling a more sudden transition. Progressive on the other hand is designed so that the comfort layer blends in with the lower layers more and there is less of a sudden transition between them. They are really opposite ends of a spectrum in terms of different layering patterns and most mattresses fall somewhere in between.
There is a fairly big difference between 6 + 2 and 6 + 3 because layer thickness can be just as important as layer softness. Because a 2" comfort layer may need some help from the lower layers with pressure relief for say a side sleeper, the lower layer may need to be a little softer than with a 3" layer. On the other hand if a 2" layer provides good pressure relief for say a back sleeper then a 3" layer of the same ILD would be less desirable and slightly reduce the ability of the firmer layer under it to produce the best alignment. There would be more foam than was necessary so the heavier parts could “sink down” further into the mattress relative to other parts of the body in other words.
One difference between 3+3+3 and 6+3 of course is flexibility in layering possibilities. You can adjust all 3 layers with the 3+3+3 while in the standard version of the 6 + 2 you can only adjust the support layer. While both can work very well … with a 3+3 you can effectively increase the support factor of the lower layers over a layer that only has a single ILD. For example if someone was to choose a 28 middle layer and then a 36 bottom layer … the rough equivalent would be a 6" layer of about 32 ILD. The 3+3 would start off softer (could help the upper layers more) and then get firmer faster as you compressed deeper into the layer while the single ILD 6" layer would “firm up” at a more gradual rate. One is not “better” than another but the 3 + 3 is more flexible. They both would involve different choices.
This would depend entirely on the person but for most people no. The layer thickness would make a bigger difference here than the layer ILD. The 3" SleepEz soft is 22 - 24 and the 2" from mattresses.net is 22 so they are pretty much the same. With SleepEz you have the “standard” option though of choosing a firmer comfort layer which could offset the thicker layer for a pure back sleeper if needed while the mattresses.net thickness is already in the range of being good for a back sleeper and would be “adjusted” for a side sleeper with changes in the layer below. Both can make custom adjustments to their “standard” layering choices if requested.
Only if the layer thicknesses were the same and the 2 bottom layers of the sleepez were both the same ILD as the 6" layer.
In effect they are different constructions and are somewhat difficult to “translate” into each other although there are versions of each that are closer together and further apart. The most accurate method is to closely approximate a layering that you have tested or without testing to go by the suggestions of each vendor that have proven to be the most accurate “on average” for people with a similar profile. They both have a lot of experience with many different people and sleeping styles and will willingly share it.
I haven’t seen each cover in person and directly compared them but to my knowledge they are both good quality options that would roughly compare. You could ask them how many oz/sq yd of wool was in each to do a rough comparison of the wool content as one means of comparison. I do like the sleepez non quilted cover (the picture on their site is not correct and you can see it here … except it’s off white not the color in the picture).
IMO … yes to needing it … and what type will depend on the tradeoffs that you believe best fit your circumstances. Post #2 here will give you some examples of some of the tradeoffs involved. A search on “protector” without the quotes will also bring up a few more posts. This article also talks about some of the choices involved in the quilting. Every layer on top of your mattress will have some effect on the layers below it and the thicker and less stretchy the layers above the latex (quilting, ticking, protector, any mattress pads, sheets) … the bigger the effect it will have on the latex.
The goal is for your foundation/bed combination to be as stable, firm, and non flexible as possible regardless of the configuration you use and in the case of a double or larger size there should be good strong support to the floor in the middle area to prevent sagging. Slats should be a maximum of about 3" (or less) apart and the construction of the foundation should be strong enough to not bend under the weight of the mattress with the people on it. Some manufacturers sell a steel foundation on a wood base which is also very strong and seems to work very well. If you can find a suitable foundation locally that is strong enough and has slats that are close enough together then that is a great option.
Hope I managed to answer all your questions.
Phoenix