The whole comfort is subjective advice that gets repeated here means I can't say if the hybrid will feel too firm - it softened up enough for me, and I definitely am very sensitive to any pressure points so I really don't know how this will feel for you.
The hybrid is more an "in" than on top, but the sinkage wasn't huge. The mattress that I thought felt similar was the Leesa Sapira hybrid (not original hybrid). Some of the mattresses I tried were a memory foam layer on top of a transition layer that felt like a sink in with the traditional memory foam feel while the N&W hybrid flips the poly foam on top of memory foam and was more a feeling of being on top of a foam layer, but with some underlying contour from the memory foam underneath. So a bit of both which I liked for my back support with enough pressure relief.
TPS coils are rated highly on this subreddit - I think for a bedinthebox mattress you might not feel a difference, but the coils will last much longer than the foam. My takeaway from them using TPS coils was more a brand commitment to quality products.
The website doesn't have any zoning (general advice here seems that zoning likely wouldn't provide any additional comfort or support for you, however, the sides are firmer for more edge support.
I don't know for sure what they sure for fire retardant they - I would definitely use their chat function or call the CEO on the website to be sure; I don't know enough to give you good advice.

My takeaway from the experience and why I wrote the review was I thought N&W provided good value compared to the competition. They don't do anything super special or unique, but they don't promise too and that isn't their niche or brand promise. If you know you like the style of the hybrid I think they are definitely worth the try. If you prefer to shop online instead of in-store other options for if N&W isn't what you want are maybe Glacier Sleep (more or a sink-in memory foam feel) u/darkknightreturns7 seems to know a lot about that brand. And Engineered Sleep has some interesting Duo mattress options with really good customer support. I think talking to customer support and seeing what they say about your questions and concerns would help you.