1. Use the W2 amount. You are correct that too much time has passed to correct the W2.
2. and 3. The California return should be done first. Always do the non-resident (or part year resident as your W2 has made you) return first and then you do your resident return to take your credit. NY does have a maximum allowed credit so you may end up spending more in California than NY gives you credit for. But ignore Chat GPT on this one and do the non-resident return first.
4. No. You'll need to re-file any forms with New York that have changed numbers. Because of the California and RSU income that could be a lot of them. And California didn't get an original return since you didn't live there. So you will need to file a complete tax return with them.
5. You can't efile a return that is more than three years old. You're going to have to paper file all of these.
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