Tax rules for APRC

If someone has not filed taxes for previous years, could that person file back taxes to meet the 3 year proof of income for an APRC? If you can file back taxes, can the income be claimed as “private tutoring” or something similar and continue to be filed that way for future years?

Also, if a couple files jointly on their taxes or show a yearly deposit to satisfy the income requirements and want to change a JFRV to an APRC…does that mean the APRC in any way is tied to the marriage? Or is the PARC completely 100% independent and unattached from the marriage?