Hi houston101,
You can read more about the different types of latex allergies in post #2 here. Type IV latex allergies aren’t a significant issue with latex mattresses because they are a contact allergy and there is no contact with the latex in a mattress.
Latex can also be beneficial with other types of allergies which are much more common such as dust mites and their byproducts and you can read more about this in post #2 here.
Answering in more specific terms would depend on the specific allergies or sensitivities that were most likely to be an issue with your daughter (and this may be a matter of allergy testing) because no matter which materials you choose there is always the possibility of an allergy or sensitivity to it.
Mattresses that are made with innersprings and natural fibers are much less common than mattresses that include some kind of foam but I don’t have a list of what each manufacturer or retailer makes or carries so outside of their websites you would need to call and talk with them to find out if they can either make (in the case of a manufacturer) or order (in the case of a retailer) a mattress that isn’t listed on their site that has the combination of materials you are looking for. A quick scan of the Houston options didn’t show anything like this on the list but there are several manufacturers there that may be worth asking. There is also a list of some innerspring / natural fiber mattresses in post #4 here and you could check if they had a retail finder on their site or call them to see if any local retailers carry them.
This would depend on what you were trying to “protect” against or on your answer to the question “how safe is safe enough for me” because both of these can only be answered on an individual basis (see post #2 here). A mattress encasement (not just a protector or mattress pad) can protect against particles in the mattress such as dust mite feces, pollen, dander, and other small particulates that can escape from inside the mattress itself but it won’t protect against any offgassing or VOC’s that come from the materials themselves because these are gases. You can read more about mattress encasements in post #2 here. It’s normally much easier and more effective to avoid materials that you suspect your daughter would be sensitive or allergic to in the first place but as a worst case scenario you could also encase the mattress in fairly thick polyethylene plastic (5 mil clear plastic) to protect against VOC’s but this would also quite uncomfortable and hot (see the first part of post #2 here).
All of this can be so complex and involves so many unknowns it can only really be answered at the level of individual “best judgement” or “personal belief” because in a modern world where synthetics and other allergens or materials that some people are sensitive to are all around us there is no way to really answer these questions with absolute certainty to all people’s satisfaction.
Phoenix