To factor in a future extension of past lifetimes by adding a new top row onto my Soul Matrix diagram, do I also need to add a new bottom row representing future extensions of future lives?

Indeed this is so. And as you have done so many times, given the merest hint of something, from your creativity you will continue to question and probe and think deeply and will often extend those gains much further, to see a logical next step or a conclusion when close to having things finalized but needing one more push forward. This you do with your boundless creativity and energetic zeal for understanding everything you can and pursuing the truth to wherever it leads. So this would complete the matrix quite nicely in having this extension to further its reach and this will go a long way to minimizing complications coming from new difficulties arising from either deep past or deep future to circle back and stir up problems once again and thereby represent a setback or a reversal of healing gains in some instances. This will be powerful insurance to limit such events to the most extreme of circumstances, and this will add significantly to the effectiveness of all the healing sessions you do.