Shivab, I'm glad you like the question. I agree that there isn't much information about this, and also that a taxpayer might be able to prevail on a state tax authority in using another reasonable method based on the facts and circumstances. Yet not straightforward in light of employer's W2's specifying a particular apportionment. Uphill battle for the taxpayer to oppose the W2.
I filed a request for General Information Letter with Colorado, a state where I have never lived, but which has high-functioning GIL process. Their response was entirely unhelpful to my position, but at least gives a preview of how state tax authorities see this. https://tax.colorado.gov/sites/tax/files/GIL%2020-004.pdf
I'm no expert on options, and have never received an option in my life, so I won't try to presume how options would be treated. RSU treatment complicated enough for me!
