Originally posted by: Leprechaun
More or less, the drama followed the novel closely.
Fan Changyu's arc felt a bit exaggerated and fast-forwarded in the drama. She didn't become a general just after stepping out of the carriage, like in the drama. She enlisted properly (didn't want to enlist under Xie Zheng, so she enlisted under old General He's army), started as a foot soldier, promoted to a commandent, and gradually became a general. Her plus point is her brute strength and uncertainty on the battlefield. All her attacks are labelled unpredictable. So it was easy for her to catch the opponent off guard and use it to her advantage in those few seconds. Later, Xie Zheng taught her combat skills that could channel her strength in the most useful way (at that time, she was still a commandant). I felt her journey was way too smooth in the novel with all these extra details, I'm not surprised that you found it odd in the drama 
Ohhhh yea I forgot.... Xie Zheng annotated many military manuals for her to learn, so yea~ that's how she got her idea of arts of war!!!
Regarding her general arc being a step to match Xie Zheng's status - as shown in the alternate ending, if all these things didn't happen, she would have been born and brought up as a noble lady, so she claimed what should have been hers.
TMI: There's this rule that any official with a rank could only marry a daughter whose father was also a ranked official, if otherwise, the girl could only be a concubine, not the primary wife. Thus the line from Song Yan when he told he was willing to take Fan Changyu as his concubine. So was Fan Changyu made to claim her identity just so that she could be a Marchioness?? - Maybe.... but not entirely.