I think the idea is great and would have all of the benefits that you mention. However, iffen you don't jump on that foam you will miss out on it and the point will be moot. (Sorry, reality check.)
Then there is the practical side of things. Does the benefit justify the effort? I have learned that, while the process (pick one) is not especially difficult, each step or process adds time, material, complexity, and time. I know, I know, you have time, but time is effort and effort takes effort and material, and material is money; doesn't matter how much... it adds up.
Would it not cost about the same or less in materials and be less effort to just paint a few leveling coats of cheap wrong color paint on to the slightly rougher foam before applying the canvas coat?
I'd love to see you try your kraft paper method in test and/or full scale, because I think it would be neat and sturdy and unique, but (reality check) I don't think it will be simple or thrifty, in the end, or an efficient use of time (i.e. you will be exhausted after the effort...everything sounds simple until you do it).
I know, it sounds like I am the doom sayer, but I'm slogging it out in the heat and humidity and nothing is simple or thrifty enough for me to consider adding a layer of... of... of everything; material, application, time, effort, repair of flaws (there will be flaws), material, application, time, effort, repeat.
(Reality check; did I mention that I am at that point in my build where I am wondering if I will ever get done?)
Sorry, don't mean to be a downer. Just my $.02 in the state I'm in at the moment. Take it for what it is.
The decorative Paper-mâché interior must absolutely take time, but it is highly decorative and therefore, IMHO, has merit on that basis alone.
(Surgeon General's warning: Adult beverage was involved in this reply

, no offense intended.)