Hi tofues,
Welcome to the Mattress Forum! 
While I can’t comment upon the potential appropriateness of a mattress configuration for your personal comfort (there are too many unknowns, variables, and personal preferences involved in choosing a mattress for someone else), I’m happy to take a look at the componentry you’re using and comment upon that.
If the slats are 4.5" apart, then this would be close to the maximum that most manufacturers recommend for a mattress using a polyfoam base. One concern I would have would be the strength of the wood used for the slats. Even in the closeup photo for this item some of the slats are bowing inward, so that is something I would keep an eye on as time goes by using this base.
This is a high quality cushioning material in a “softer” ILD. Celsion is just the trade name for the inclusion of a small amount of phase change materials within the latex. The feel would be the same as any “normal” Talalay latex.
Whether 100% NR of blended (SBR/NR) Talalay, both styles will be quite similar in “bounciness” and resilience (most people wouldn’t be able to tell the difference between the NR or SBR/NR in the same ILD in a finished mattress).
Most latex you are likely to encounter (NR, SBR/NR or SBR) will have some sort of third-party certification against harmful substances and VOCs, such as Oeko-Tex or Eco Institut. There is more about the different types of organic and safety certifications such as Oeko-tex, Eco-Institut, Greenguard Gold, C2C, and CertiPUR-US in post #2 here.
It is true that side sleepers in general prefer a bit more surface contouring, allowing the shoulders and hips to sink into their “comfort cradle”, while at the same time still maintaining their overall alignment. A slightly softer comfort layer on top of the mattress is a popular choice, as long as it is combined with materials beneath appropriate enough to work as a unit to provide adequate “deep support”.
I think you’re not correctly understanding what the Celsion is in the latex. It’s simply a phase change material that’s added to assist in temperature regulation (although this will tend to have a temporary effect on temperature while you are first going to sleep until temperatures equalize but have less effect on temperature regulation throughout the course of the night), and is not something that takes away from the traditional “buoyant” feel of latex.
This would also be a high quality and durable material, and choosing to go with a slightly firmer Dunlop latex as your “transition” layer is a very popular “progressive” design combination.
Ideally I recommend at least 1.8 lb in a polyfoam core, unless there are other considerations such as price, but with your lower BMI this may work out just fine for you. The concern I have with this product (I’m not personally familiar with Ritchie Foams) is the description of the foam as a rubber, which it is not, so that gives me pause as to what other inaccuracies there may be in the description of their product. I’d suggest a detailed phone call with them before placing an order to confirm the actual product you are receiving.
There would be a difference in feel and durability of this foam compared to Dunlop (the Dunlop will last much longer and be a more supportive material), but some people do prefer the use of a polyfoam core for either the cost difference or difference in comfort.
This would work well as a mattress protector, but I’d recommend an actual mattress cover that surrounds all of your foam layers, and then use the mattress protector over this. There is a list of some mattress cover suppliers in the mattress component post here.
Phoenix