I've asked hotel concierges to order (with delivery) tickets for me a handful of times, with service fees ranging between Y10 and Y50. Y50 might seem a bit steep on the surface for those of you who use SF Express regularly, but is worth it to me.

Off site ticket offices are still my sales channel of choice. As noted up thread, they are less common these days, but they definitely still exist. Just find one that is convenient wrt your hotel and/or places you intend to visit.

Assuming you book a few days in advance, and it's not a busy travel period, I wouldn't be especially worried about availability on Beijing-Xi'an.

Yes, G trains are the fastest. Single digit trains (none on this route afik) have the fewest stops, followed by double digit and triple digit trains. There are also still some slower overnight trains on the schedule, which I prefer sometimes when I can get soft sleepers. For these, the pecking order is typically Z, T, K, followed by trains without a letter prefix. Unlike G trains, some slow trains actually have edible food.