As you will notice from your review of the threads, I am an X-Tol R devotee.
Your suggestion about using the old film to season the working solution is fine - just adjust the time gradually between the shorter times for using fresh stock and the longer times for using replenished developer. A straight line approximation to the necessary time adjustment should be fine.
You could probably just do your regular photography and do the same. The results will be sufficiently similar that you should be happy with the results throughout the seasoning process.
While I see some subtle benefits to the image quality, the biggest benefits for me arise because of the ease of use and economy.
Whether or not you use inversion or rotation agitation, I strongly advise always using the same volume of replenished X-Tol in your developing tank, whether or not you have 1, 2, 3 or 4 rolls in there. That will help maintain better consistency over time. As you are using it replenished, their is no economic downside to doing so.