Changsha is the capital of Hunan, which is a central southern province of China. This drama is set in 1939 at the prelude to the first Battle of Changsha and it was also the beginning of WW2. There were actually 4 bloody battles in total, where the Japanese tried and failed to take the city. It is significant because this prevented the Japanese from consolidating their territories in Southern China. It was one of the important achievements the Nationalist Army of Chiang Kai-shek's Republic of China. The CPC, which was founded in 1921, did not come into power in China until 1949. Xue Yue (who is on the cast list) is a real historical figure and the commander of Changsha's defense at the time.
The rest is plot speculation and there will be spoilers:
During this time, the Japanese already controlled Shanghai via their collaboration with the Wang Puppet Regime but in Changsha, the KMT is still firmly in charge and while the Japanese may have moles in lower or even mid-level positions, they definitely should not be in direct control of the NBIS or Changsha's military defense operations. If they were in control of Changsha already, they don't need to try to invade it. So Kunwu is more likely to be someone hiding among the mid to lower ranks of the NBIS Hunan Station or the military defense operations.
We can assume that the CPC's role in this narrative will be to "save the day" behind the scenes by helping to weed out the hidden Japanese moles within the KMT. This puts both Yang Jiming and Assistant Wu in the good guy camp. They are all hidden CPC spies. At the time CPC were the underground rebels or freedom fighters depending on your point of view, so anyone being part of their organization kept it a huge secret.
Muzhi's secret is simply that he is also a hidden member of the CPC and he was grooming Hongshen to join the CPC as well. Most likely his father before him was also a CPC member.
Kunwu's painting was for Xu Meijia to send back to Japan. It is a message to his 3 family members that he will soon reunite with them. That suggests they are not in Changsha with him. Its just a red herring, no need to try to overthink who they are.
It is very possible that Kunwu has not even showed up onscreen yet.
I will update as appropriate as we go along.
