Hi RoboLobo,
You are asking some good questions which is great to see 
As you likely know better than I do … fibromyalgia can be particularly challenging and also doesn’t have a consistent set of symptoms. Overall though … I would tend to increase the emphasis on pressure relief if in these cases but it’s also important not to go too far in this direction because hyperextension of the joints can also cause pain and discomfort and alignment is still an important issue (which means enough but not too much thickness and softness in the top layers of the mattress).
I would also choose between latex and memory foam in my comfort layers. Latex is a higher resilience material but both are similar in their pressure relieving abilities (comparing most memory foam to softer latex) although in some cases the higher resilience of latex may mean that a top quilting layer or even topper of a lower resilience layer (foam or natural fibers) can also be beneficial (high resilience materials in the very top of a mattress may be uncomfortable for some).
In the many calls I make each day I haven’t really tried again recently but I would guess based on other similar feedback as well that it’s probably the same. they tend to leave their retailers as their customer interface.
As with others who don’t have fibromyalgia … the choice of memory foam is a personal preference but many people that do have fibromyalgia do well with it. My only caution (related to my previous comments) would be to avoid the temptation to go with layers that were too thick/soft because of the risk of joint and spine alignment issues that could be involved and to take into account that it will soften initially and further over time as well.
I would also make sure that with memory foam that the density was suitable for her weight (for durability reasons) although it may be a good idea even at the risk of lowered density to include some softer 4 lb memory foam in the mix (perhaps 2" over a layer of higher density memory foam).
With latex I would also make sure that the ILD of the latex was suitable for her weight because higher weights will perceive firmer foam as being softer than someone who was lighter and going too soft with higher weights can allow too much of the firmer lower layers to “come through” . Sometimes slightly firmer comfort layers can actually feel softer if they do a better job of isolating the person from the firmer layers below them.
Overall … both can do a very good job and I would leave the choice to preference rather than “better worse”.
As you know I think very highly of Rocky Mountain Mattress in terms of both their mattresses and their service and guidance on the phone. They could certainly make a good choice IMO if you were comfortable with an online purchase and some of the extra risks involved. They will also custom layer a mattress if that should be a desirable option. I would tend to first make a mattress purchase without any additional topper (whether with latex or with memory foam)) and then use actual experience on the mattress to decide on what if anything needs to be added for fine tuning. I think this is a much “safer” approach rather than buying two untested components together. Smaller steps done one at a time with some good evaluation in between each step or addition is usually a more effective approach IMO.
There are no factory direct manufacturers in Santa Barbara with one exception that I would not put in the “typical” factory direct category but there are some better options in the area which are listed in post #2 here.
There are many manufacturers in the greater Los Angeles area which are listed in post #2 here. I would call them first to talk with them about your needs and preferences to help you decide whether any travel time would be worth it based on the phone call and on the level of knowledge, service and interest they took in helping you and on the options they have available for testing and purchase. There are some very good choices available in the general area.
I think you are on the right track and asking all the right questions so with some time, testing, and some careful evaluations I think you will do very well
If you have questions along the way feel free to post them in the forum.
Phoenix