Q: "The properties were never foreclosed upon, just written off. How would those be handled?"
A: All secured claims (mortgages, liens, and other security interests in real or personal property) should be listed in Schedule D.
It's highly unusual (and probably not the best practice from a creditor's perspective) for a lender to issue a 1099-C before the property is actually foreclosed and sold for an amount less than the outstanding mortgage balance. Depending on what you want to do with these properties, these 1099-Cs may actually be helpful for someone in your circumstances. As a hypothetical, let's say you want to keep one of the investment or rental properties you own (not including your personal residence). Assume that the outstanding mortgage balance on this property is $200,000, and the lender filed a 1099-C on this property for $50,000. That 1099-C could potentially be used in bankruptcy court as an admission by the lender that the property is only worth $150,000 ($200,000 - the $50,000 "written off"), and you could possibly cram down the mortgage to a total of $150,000 (provided you can pay off the entire $150,000 through your Chapter 13 Plan).
Regardless of what you ultimately decide to do with these properties, your case is going to be at least somewhat complex due to the debt limit issue and tax issues involved. This is not a case you want to handle pro se; I would strongly suggest you hire a local attorney to assist you with your case.