Basement wall

I have a 100 year old farmhouse. When purchased 2.5 yrs ago, the attic was not well ventilated (sloped roof, not flat). There was much mold/mildew on the existing attic interiors opposite the roof. Note that the roof and shingles seem ok as new plywood was added when shingles were replaced about 5 yrs ago. No mildew on plywood,or attic floor rafters, only on original roof boards. Added ventilation 2 years ago. There was also loose fill cellulose as insulation between floor rafters, all exposed. The mildew stains still remain on the original roof boards two years later. Question 1: Should I do something about the mildew stains, and what should I do? Question 2: I would like to add, or substitute newer insulation (like mirafil) in the attic. Should I be concerned about the pre-existing cellulose loose fill insulation since it was exposed to the poor ventilation (i.e. high humidity) and mildew/mold? Should it be removed? I do not notice any mildew or mold on the loose fill. Thanks!

I am planning an addition to my house. One contractor uses a technique called "slab overlay" in which a trench is dug for the footings and the interior is excavated only enough to allow for a two-inch layer of concrete and the floor joists. There is virtually no space between the concrete and the joists, except in the middle where a trough is formed to allow for the ductwork to run under the joists. Another contractor would excavate the whole area down to 42" and provide a crawl space. He says that trench foundations are "bad construction". Are his concerns about trench foundations valid or is he just bad mouthing the technique because his competitor uses it? If it matters, I live in the Chicago area so cold is a concern. Thank you in advance, HomeownerJ