Hi, Mr. Herbert, my pleasure to help. 2 days in Xian, the city I am living now, are enough for your to see Terracatta, City Wall, Famensi, Big wild Goose Pagoda. Drum and Bell tower is just in the city center, not necessary to climb up if your time is tight. Just take a photo of them. 3 days in Beijing, Forbidden City and Tiananmen Square, Temple of Heaven , Yonghegong and Guozijian, Great Wall and Summer Palace for one day. It is a good idea to arrange three one day tours in Beijing, because it takes some time to arrange the tranport your own. Then it can save some time for your to see more. Comparatively, Beijing has more places to see. Here are some Beijing one-day tours(https://www.travelchinaguide.com/package/beijing-one-day-tours.htm) provided in this website by TravelChinaguide. For your Xian tours(https://www.travelchinaguide.com/package/xian.htm), it may be difficult for your to travel Famensi and Terra Cotta because of the transportation problem. Other main attractions are in the city proper and easy of access just by taxi. If you stay in the hotel within the city wall area, you can just walk around to see the Bell tower, Drum Tower, City Wall, Moslem Street, City God Temple.
As to the flight deplay, yes, your worry is reasonable. The recent weather condition is good. I guess it will be fine when you visit. This winter is warmer than last winter. When you make inquiry and booking though this travel service, you may ask if there is backup plan to assure your travel in case there is a serious deplay.