Hi mjl5007,
I would strongly suggest reading post #2 here before you consider ordering from FBM. I personally don’t believe that you are likely to receive what you believe you are buying. There is really not much point in ordering 20 ILD talalay from them when it’s highly unlikely that this is what they are selling unless of course the ILD or type of latex you receive is not a critical issue.
There is nothing more frustrating when you are trying to put together a mattress that really works for you than having to deal with a piece of what turns out to be unknown foam that isn’t doing what it’s supposed to do.
Their 30 day return policy excludes shipping (both ways) and also has a 25% restocking fee which could eat up much of your purchase price … although you would at least have part of your purchase price refunded so this is better than no refund at all.
I would go with the cotton/bamboo cover over the terry cotton/polyester covers not only because it’s a better material but it has a lower price.
I know this seems harsh but I personally have a great deal of trouble dealing with outlets that I don’t trust to supply me with what they say they are selling or whose quality and integrity is questionable at best. For me it’s just not worth the headache … no matter how cheap they may be (and in this case the savings are not so significant anyway).
Without knowing the layering in your current mattress it’s difficult to be exact but assuming that the foam in your mattress is firmer than normal I would say the odds are good that a 2" topper in the range of lower 20’s (like you are looking at) would work well. Thicker than that and you would be risking alignment issues on your stomach and thinner than that would risk inadequate pressure relief on your side … and I think that good pressure relief will be especially important for your wife over the next few months 
Phoenix