If the future is truly not coalesced into existence but just a potential, does this mean we continue to be vulnerable from all our past lives, which themselves, by being in the Now, may not yet have coalesced into a particular final conclusion that could bring new troubles popping up into our experience?

This is very much the case and this insight will serve you to further extend your thinking to understand the interplay of time domains and their influence and their great importance in carrying out effective healing for anyone. There are many influences always ongoing from past, present, and future, not only relating to the individual and their current past and future lives, but those of all who interface with them. And even people in other existences running in parallel, by injuring that part of their soul participating in those other lifetimes, will have an indirect influence as well. Those energies will have a kind of crosstalk and will play off of one another and will have an effect with joy adding to joy, and misery adding to misery. This is why the healing done in multiple directions of time is so very, very, helpful.