Hi Gdub42.
Welcome to our Mattress Forum. 
To jump right into your questions…
There are a few factors at play here and it is good to see you are considering experimenting in view of an all latex mattress purchase. As the “experiment” OMF mattress will not be yours in the long term, there are different factors to consider. First off, the 2 very different support systems (knotted offset coil vs all latex) will respond very differently so the coil unit experiments cannot be used as a baseline for purchasing an all latex mattress. Being very athletic you are likely are carrying more weight in your torso and shoulders so you need a mattress that allows your shoulders to sink-in adequately and to conform to your body shape when sleeping on the side.
• One area of concern is that the innerspring in the Orthopedic has a more specialized design (along with the other layers above it) and any experiments on this unit would not be relevant to an all latex mattress. The coil is a lower gauge (stronger) and is knotted rather than free arm (or the more responsive Pocket coil) so overall it would be a more solid, stronger, and firmer support core. In addition to this … different types of innersprings have different properties and response curves and are used for different reasons and purposes in a mattress. Some are designed to contribute more to the comfort of the mattress along with the support (more dual-purpose) while others are used more for the support and the comfort is created through the combination of more specialized materials and foam above the coils. For example, … a very firm foam may be suitable for a mattress base layer but would be less conforming than a softer base foam so would need a different combination of materials and foams above it. A support core such as latex or pocket coil, that is softer and more conforming would need less foam on top to create a similar feel and would be less specialized in each layer. So, the coil in the Orthopedic line is more “specialized” to provide higher levels of support and the combination of layers above it are where the feel and pressure relief of the mattress is designed into it.
• The (12.5") luxury firm version OMF you are considering has 2.5" of foam (density and IFD unknown) on each side of the 8" knotted offset coil innerspring unit. While it is impossible to predict how this will work for you in terms of needed support/comfort and preferences, this unit would certainly better accommodate a side sleeper provided that you zero in on an appropriate thickness and ILD of latex topper. However, your large width differential “with almost no hips but broad shoulders” may still present some challenges. In terms of “versatility” to use without a topper for a child, this depends of the age of the child and some other factors I’ll outline below in my response.
• The (10.5") luxury extra-firm version OMF you are considering has just 1.0" (density and IFD unknown) on each side of the 8" knotted offset coil innerspring unit. This bed is certainly designed with stomach & back sleepers and very young children in mind. What you feel on the mattress in terms of PPP (Pressure relief, Posture and alignment, and Personal preferences) would be more important than the specific differences in design.
Children have much different needs than adults because their body shape in not yet as curvy and they tend to do better with a firmer surface. I personally would focus on finding the appropriate comfort/support balance for you whether you decide to go with a customizable mattress or mattress/topper combo. One of the advantages of getting a multi-layered component style system mattress is that you can add or exchange layers as your child grows. If and when they need a softer comfort layer in their “sleeping system” you can also add a softer topper to any firmer child’ mattress when they get older and develop more adult proportions. Generally, a child won’t need more than 6" or so of latex, so you can certainly use the lower/firmer layers for a child’s bed as they are very adaptable at a younger age and can sleep almost on anything.
Generally, for growing children, you want something more in the “medium” to “medium-firm” range, due to their epiphyseal plate and postural formation.
Post #2 here includes links to most of the better forum posts and topics about mattresses and children and includes some suggestions and guidelines and links to some good quality/value options as well. This mattress uses good quality materials but I would tend to choose a firmer top layer because children tend to do
To address your question of whether a firm or extra firm mattress would be a better candidate for a topper. Adding a topper to soften a too-firm mattress is very common way to fine-tune a mattress when done properly. The opposite is generally not true and you cannot fix a too soft or sagging mattress by adding a firmer topper.
Side sleepers have more gaps to “fill in” … additionally your “board shouldered” profile requires a mattress that is more conforming than that of back and stomach sleepers. The medium Talalay would be a better option for this application (but the firmness choice also depends on the topper thickness and the support/comfort layers under it) The benefit of latex is that even in the softer versions, it has a higher resilience and is more “supportive” than other foams, and will both allow more sinking-in and help support the more recessed areas of your body, such as the cervical and lumbar, that need “filling in” and are not in close contact with the firmer support layers underneath.
I’d keep in mind that how far the shoulders and hips “sink-in” has to do with how much weight it carries and the surface area that carries the weight, Thin or narrow shoulders will sink in deeper than thicker wider shoulders at the point of contact when on the side. But how deep it sinks will also depend on the BMI in that particular area. In the same way …narrow more pointed hips will sink in deeper on the side than when lying on the back on a larger surface area. This is part of the reason that weight and body shape are so important and that they are matched to a mattress. Typically, it’s very unusual for the shoulders to sink in too far and the hips or pelvis not enough … but in your case, this could certainly be possible so you may need to consider having a softer zoned under the shoulders and firmer under the hips.
While no one can tell exactly what firmness would be most appropriate for you (as only you can feel what you feel on a topper/mattress combo), I would agree that something on the medium to softer side will be more comfortable and appropriate in your case than something firm. Broad-shouldered and thin hipped, also need to find the right combo of comfort softness and pillow height to allow the shoulders to sink into the mattress enough to keep them into alignment with the hips without “cranking” the neck.
Once you’ve had a chance to consider the additional variables I pointed to, let us know of additional questions or any new updates as you move forward.
Phoenix