What's the difference between nonstationary and time varying?
My understanding :
Non-Stationarity is mentioned in statistical sense where channel coefficients are non-stationary random processes. You can look for Wide-Sense Stationary Processes and Strict-Sense Stationary Processes and their definition and properties.
Time-Varying is with regards to time-domain channel impulse response characteristics. Time-variance indicates that depending upon when any specific input is provided to the channel, the output will change because the channel is varying with time.
A particular channel model can be time-varying but wide-sense stationary. This would mean, depending upon when any specific input is provided to the channel, the output changes but the statistical properties of the output like correlation function, mean etc will follow wide-sense stationarity conditions.