Here are two user-friendly, reputable agents with English websites where you can check availability and book tickets for domestic China trains right now:
www.travelchinaguide.com
www.china-diy-travel.com
Yes, for you first class seats on the D and G trains are worth the money. I'm glad you mentioned your height, since IMO overnight sleepers like the one you plan to take Beijing-Xi'an may not be comfortable for 6-footers--I will defer to other forum members who are closer to you in stature than me, and who have experienced Chinese overnight sleepers. If you reconsider, then on 7/4 take the late afternoon G669 Beijing West to Xi'an North 17:27-22:52 and spend the night in a hotel bed in Xi'an vs on the train. 1st class seat still available.
Yes, Guangzhou South is far from central Hong Kong, a few hours by road when you figure in the land border crossing/Immigration. There are shuttle buses that run from various points of Guangzhou city to Hong Kong Airport; if you decide on this method, we would need to give you further advice on the best location and timing. Skylimo or a private/shared van transfer would be more expensive but in combination with the train ticket, still less than flying Guilin-Shenzhen or Guilin-HKG (and the latter nonstop is not available on the morning of the 9th nor the evening of the 8th).
I don't think taking the D2367 on the 8th gets you anything except time in the Guilin region so shortened that you might as well not bother at all. Frankly, with the time you have, you are trying to fit one node too many in the mix. You could reconsider, cut Guilin out, and move that time to the Shanghai region where you could see Suzhou, a water town, etc.
Alternatively, if your time in Hong Kong on 6/25 to 6/27 is purely personal and you have no business commitments, you could cut back by 1 to 1.5 days, get going to Shanghai earlier, then add the time back to HK at the end, giving you plenty of cushion for that intl flight out. We'd have to check the new days of travel to see what's available for trains and planes.