Hi Michael,
As you know you certainly made a great quality value choice but that of course doesn’t mean that you will sleep well on a mattress and I’m sorry to hear that you (or your wife) are having issues with your mattress.
It’s unfortunate that she didn’t mention the issues she was having earlier but you still have many options available to you although it sounds like she may not like the “feel” of latex and may prefer memory foam anyway in which case there may not be many options available that will satisfy her because latex certainly doesn’t “feel like” memory foam and if she has a strong preference for memory foam then she may be predisposed to return the mattress anyway no matter what happens.
There is some information in post #2 here about the most common symptoms that people may experience on a mattress and some of the most common causes for them that may help you identify the reasons for her experiences and "symptoms.
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
The first thing I would suggest though is a phone call to SleepEZ so you can take advantage of their knowledge and experience with other customers that are similar to you that have had similar issues since you may not be making the type of changes that would be the most likely to help with her “symptoms”. They will be the most reliable source of suggestions that can help although if your wife is predisposed to memory foam then even if the suggestions are successful she still may not want to keep the mattress.
The most common cause of lower back pain is an alignment issue that comes from either support layers that are too soft or comfort layers that are too thick or soft.
The incremental change that I would normally start with in her situation would be to exchange the medium and firm layer in her side to “firm up” the support but allow her to keep the softer sleeping surface (from top to bottom this would be S/F/M)
After this if it was necessary I would also try exchanging the top two layers which would give her a slightly firmer sleeping surface (from top to bottom this would be M/S/F).
There are many other combinations that would also be worth considering that could firm up the support even more (including from top to bottom M/F/M or S/F/F) but I would base any further changes on the results of these first two based on what you “learned” from any changes in her “symptoms” between different configurations.
Again though … the first step when you need to make any changes is to have a more detailed telephone conversation with a knowledgeable and experienced manufacturer such as SleepEZ so you can take advantage of their knowledge and experience to make the kind of changes that have the best chance of success because most consumers don’t really have the knowledge or experience on different layering combination to really know the type of changes would be most likely to work on their own.
They may also be willing to extend the trial period to give you a little more time but I think this would only be worthwhile considering if your wife would keep the mattress if her “symptoms” were resolved since it may not be worthwhile taking the time to find the best combination if she really doesn’t want to sleep on latex anyway.
If there is a ridge in the middle it may be because the layers aren’t laying flat (latex is very stretchy and floppy and you need to “wave” and “flick” the layers into position so they are laying evenly and flat) or it could be from the foundation underneath your mattress if you have a twin foundation where the sides of each foundation where they come together in the middle are firmer and the middle of the foundations have some flex. Other than this the bigger the difference between the sides the more you may feel the transition between them although most couples would say that the tradeoff is well worthwhile (see post #2 here).
Most latex mattresses don’t have (or really need) edge support (see post #3 here) and for most people it’s not an issue but for a few it is and this is also related to the softness/firmness of the support layers and the softness of the comfort layers as well and of course whether someone tends to sleep with more of their weight concentrated on the edges of the mattress. For sitting it’s usually just a matter of sitting a little more towards the middle of the mattress than on the very edge. These are usually issues that are part of testing for PPP when you are trying latex mattresses.
While latex is generally “good” at motion isolation … it certainly isn’t as good as memory foam which is the most motion isolating of all the foam materials.
Hopefully a more detailed conversation with SleepEZ will be your next step along with some of the suggestions in this reply 
Phoenix