All of this is very interesting and I can toss in a couple of wrinkles (without any answers). I've worked for large companies with an international presence and offices in multiple states. One employer, years ago, sent me to Venezuela to do some work a couple of times. Once I had to show I'd paid Venezuelan income tax before I could leave, the send time I just snuck out of the country (Aruba is nice!). The company I now work for has sent me on international trips and to locations in other states to do work and I've never paid income taxes or been asked to in those countries or states. Part of that may be the IRS rules which define my place of work to be in Texas and all other sites to be temporary, but I don't know how that applies to State Income Tax rules. All interesting questions though since they appear to be getting ready to send me to either Arizona or Colorado on temporary assignment for up to a year. And to make things even more interesting, my GF (with the same company) transferred to Colorado for an assignment and she was told, by HR, that she could declare herself to be a resident of either Texas or Colorado for tax purposes. Maybe this is possible because the company has a presence in both States, but it does raise some interesting questions.
