Totally doable if you’re wanting to visit soldiers etc. That’s an afternoon but you have to get there and back as not in centre of city. Xi’an was venue for school trip on a return in 2018 so can’t claim any knowledge of organising that/ practicalities of that trip. Was ferried around by host school

Beijing, Nanjing, Shanghai all doable together depending on length of stay as vaguely over the same side of country so could do by train etc but there are regional flights. Xi’an further to centre so flight for certain I’d say.

Shanghai nice but strong western community and centre quite modern. Suspect would be good for expat nightlife etc. Xi’an I liked but many cities are broadly similar except for the Terracotta Army there. Depends on when you’d go as to where I’d suggest. Xi’an I did in 43°- wouldn’t recommend 🥵

I lived in Beijing in 2010/11. Loads to see there and easy enough to go out to visit Great Wall etc. Nanjing I only briefly passed through so couldn’t comment too much- I went there as starting point for a tour of the water towns in that region which finished in Shanghai.

Yes. Done all of these