A. You can use closed-cell spray foam that way if you want, although you should realize that the spray foam will prevent the roof sheathing from drying inward. This approach should only be used if the roof sheathing is bone dry, because it will never dry when sandwiched in this manner. Most building scientists would prefer to see fiberglass rather than closed-cell spray foam under the roof sheathing in this type of assembly.
If you're chemically sensitive, it's probably not a good idea to get it installed, but it's also not a good idea to breathe musty air from the crawl space either. In addition, your house and its contents are made of lots of materials that affect your indoor air quality. If you're really concerned about this, hire a company to test your air and tell you what you can do about it. http://y2u.be/ggLAUsiuI_o