This is lovely! Yoy probably have already figure a way, but i have an idea about the broken wall part. I would fill the in between space of the remaining mdf part with a sausage of milliput. Then I would sculpt in the rumble typically filling the brick/stone layers in fortifications. Let that set. Then on the wall part that is to be removable, I would place again a sausage of miliput between the mdf, add some lubricant and press it on the previously sculpted part of the walls, in order to create a perfect fit. In that way they will be able to easily remove the part and place it back, while both parts will have some detail in the join.
I hope all that made sense
