Hi Kansasmon,
There are two manufacturers in Wichita included in post #2 here but only Denver mattress would be in a more reasonable value range (the other sells more premium mattresses). The post also includes a link to some of the other retailers in the area that may also be possibilities. With a few calls … you should be able to find out which have some latex on their floor that your daughter could test and if they can provide any information about the ILD of the latex you would be testing.
Both of the Denver mattress “mostly latex” mattresses use 24 ILD talalay in the comfort layers which would be similar to the “soft” used by SleepEz … but they also have an additional inch of polyfoam in the upper layers which will change the feel and performance of the mattress compared to a 2" latex layer without other polyfoam in the mix. The Denver mattress lower talalay layers are 32 ILD which would also be similar to the SleepEz medium but again … because the overall layering is different, they would not be a “match” … only a guideline.
At 100 lbs … 24 ILD may be a little on the firm side for some people … particularly with only 2" for a side sleeper that is used to memory foam, but personal testing is always more accurate than “theory”. I would suspect that a slightly softer latex layer (and softer than the “standard” 22-24 soft may have to be custom ordered) and perhaps 3" instead of 2" may work a little better. If I did go with a 2" top layer … I would tend to go with lower layers that were a little softer than normal to “help” the comfort layers a bit more and better “match” her weight.
Overall though … the best place for guidance with a particular mattress is the manufacturer or supplier themselves and they are much better at specifics and guidance with the particulars of their own mattresses than the more general guidelines that I can provide which aren’t specific to all the components of any particular mattress. Sometimes even smaller differences in construction can make a bigger difference than many people would suspect.
The SleepEz special has been available for a long time (several years) so I wouldn’t worry about the sale ending although of course this could happen 
Talalay will feel a little bit softer than the equivalent Dunlop if you are sinking into a particular layer more than about 25% (where ILD is tested) but using Talalay in the support layers would put you into a different mattress lineup (The special only uses Dunlop in the support layers).
Almost all foam that is poured in North America is CertiPur certified so mattresses that use American foam would be “safe” for most people even though they may not be an official part of the CertiPur program (which costs them money). Even the few US based foam pourers that aren’t I would consider to be safe. Almost all the latex sold here would also be “safe”. All the talalay is Oeko-Tex certified and most of the Dunlop would have a reputable certification as well. If you are buying from a reputable supplier … then latex “safety” is not an issue IMO.
Since she is already sleeping on the “worst” of the types of foam in terms of offgassing and any harmful substances it may contain (memory foam), I doubt that there would be an issue with most good domestic polyfoam although “safety” is relative to the person and there are some people with Multiple Chemical Sensitivity for example that can’t use any polyfoam in a mattress (ar anything else) at all until it has aired out for months.
Given her weight, sleeping position, and circumstances … I would tend towards softer than average or even softer than most “standard” layering.
Arizona Premium Mattress also has some “all talalay” mattresses in 8" which may also be worth considering in a similar price range. They are also excellent value but they use a single 6" layer for the core and then a 2" (or 3" custom ordered) comfort layer so they are not quite as flexible in terms of layering possibilities. They also have a wool quilted cover instead of a stretch knit cover for those who prefer wool in the quilting of their mattress.
Phoenix