1) Yes you can report on the cancelled debt with or without the 1099C's.
2) It is actually income (cancelled debt) so it should be reported) if you do get a 1099C say in 2019 for debt settled in 2019, IRS may contact you and you would have to show them it was correctly reported in the year it was settled (not the year of the 1099C). Only provide this if requested however.
3) If you have letter showing it was settled in 2018, then there is no need to Amend the 2018 return. You would not report in 2020 (or if you did you would show it and then back it out saying it was correctly reported on the 2018 return).
4) I would file CA by mail and attach a copy of Federal return to it. CA starts with the Federal Taxable income.
5) You will get a list of what is needed to be mailed when you submit the returns by mail. It will not include worksheets or proof of insolvency. It will include the 982 and lump the total 1099C's (one issued and others not issued) on the one 982. I would attach a sheet showing how the total amount on the 982 (1099C's) was arrived at.
i would mail Federal and State returns.
