I think speaking the language makes a huge difference.

That said, there's no reason you need a guide for everything, and perhaps not for anything. It's fine to use one, and you may get insights, but between English, hotels telling the taxis where to go, and a very basic phrasebook plus some sense of adventure, there's little to be concerned about going it alone in Beijing and other large cities.

If I were you I would not go to Hong Kong and back, or for that matter out of China. Tokyo and Thailand are worth separate trips at another point--use this for your once in a lifetime trip to China.

You should certainly consider Xi'an. Really no reason for the Hilton concierge to handle that--you can arrange a tour in Xi'an once you're there. Give the city a couple of days--one for warriors (which isn't a full day), and one for other things. Perhaps pick one other city to go to from Xi'an, and then return to Beijing before flying home (same route?).

As moondog suggests, air travel in China can consume a lot of time, between delayed flights and uncertainty of timing getting to the airport. Figure at least half a day to fly anywhere.