Is a future lifetime of greater impact by being more definite in its existence as opposed to a temporary and very much in flux projection of prior potentials, as presumably is the case for a past or current life future extension?

The future lives, by definition, have stronger and more certain energy and impact, but when compared to the sum total of energies, are lesser in number and, therefore, in aggregate, not the most important. It is the sum total of all that is happening across time domains that influences the current life, or any of the other lifetimes, or extensions. So all have an importance, and the sheer number of past lives, as well as their future extensions, automatically means that they will have a very high significance compared to the single current lifetime and its future projection as well as other future lifetimes.