<<I am wondering if it would be worth fighting with a lawyer or simply filing that 2017 CA Non-resident return, claiming my OR tax credit, and paying the difference.>>
If you received a demand letter from California FTB, the first thing I would do is comply with the letter. If the letter gives you the option of filing a 2017 return, whether original or amended, I would do so, and claim the reverse OR credit. A penalty for failure to file would presumably continue to increase over time. Best to clean up the situation asap. Just my opinion!
https://www.ftb.ca.gov/file/did-not-file/what-to-do-if-you-did-not-file.html
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
