Considering the mind-boggling nature of time and the universal simultaneity of everything happening in the “now,” is it possible to channel people during the past to discuss lives ostensibly already lived?

This indeed is possible, as you already know, and can be very rewarding. The key is knowing where you are going and having the intention for the exercise and the learning needed to help govern the navigation in a way to arrive at a point where the information will already be at hand and known to the individual to be queried. This again speaks to the high degree of innate wisdom and intelligence of the intuitive apparatus.