Are the activities on WeChat being monitored by the admins?
No, admins can't see the activities on WeChat. Although the application had problems with protocol encryption and the way packages were delivered, it seems that they are fixed now. The applications uses RSA with hard-coded public keys and the messages are encrypted in AES.
Not at all, admins don't have the privilege to monitor or see users' activities on WeChat.
No, I don't think they can know. Although we have only IP and password, if they didn't install software, they will not know.
Wanna track Wechat messages? Just try iKeyMonitor.