Thank you! This is super helpful and interesting. The concept of "nexus" definitely clarifies things.
Given that we did travel a little (particularly back and forth to WA) while working and thus did not spend every day in MO, it probably makes sense to file as a non-resident for MO while we were still paying rent in WA, even if it is a little tedious.
The bonus piece is interesting. I was expecting that to be split across the whole year and at least a part of it considered part of the income earned in the state where you file a non resident return (i.e. the bonus was based off performance for work performed while you were in the state). Does that split get as granular as whatever state you happened to be in when the money hit your account gets to take a cut of it (if they collect state income tax)?
Let's say you reside in state A with no income tax and work a few months of the year in state B, during which you receive a bonus. What happens if you leave state B for a few days to work from state C that does or does not collect income tax (or perhaps even state A) on the week/day that you actually collect the bonus? Does any state get a piece of your bonus in that scenario?
