Hi, time seems short for you to do so many things. In my opinion, you can delete the Muslim Quarter and the Great Mosque from this itinerary. You can take tourism bus no.5 (306) from the east square of Xian Railway Station or 307 from the south gate of Tang Paradise and get off at Bing Ma Yong (Qin Tarracotta Warriors and Horses) station.
When back, take the same bus to the train station and catch a taxi to the Old City Wall. Then back to the train station.

Hi. Time seems really tight. Reading to 9:00AM for breakfast, you can have it in the KFC just across the road when you get out of the train station. Then please take NO. 5 bus (travelling No. 5 or called No. 306) to Terracotta directly. Bus ticket 5 RMB. Please do not take any other bus or any other private bus. The no. 5 bus is just in the east square of the train station.

3 hours for Terracotta is necessary if you want to have a detailed visit. When you came back. Take the same No. 5 bus then it will take you back to train station. Then you can take no. 251 or No. 611 to Muslim Quarter getting off at Bell Tower(zhong lou in Chinese pinyin, 钟楼). Then walk across the Bell tower gate and you will find the moslim quarter. Most favourite snacks can be found in Xiyangshi street and Dapiying street(西洋市街和大皮院街).

The ancient city wall can be found just in front of the train station.

Hope this informaiton can help you!