Your post included a succinct but comprehensive account of the audio assignment. I relate particularly to your experience in practicing as well as the one related to the vegetarian nutrition.
I like your description of Shetaut Neter as an authentic spiritual path allowing the practitioner to purify all aspects of his personality for spiritual enlightenment, which makes it a well balanced approach compared to other paths. In that sense, it is true as you commented that it can be used universally as the divine essence is present in everyone. It is correct that the spiritual evolution does not happen in a straightforward manner but requires repetitive actions across one aspect or across many aspects often in a forward and backward line.
I can relate to the fact that your previous experience with other spiritual paths have provided you with the discipline and mental attitude to practice at least twice a day with the consideration of moving toward a third practice. I find your question about re-using past mantras in current practice to be relevant as I was thinking among the same line and, in that sense, Dua for Sebai MAA, to have clarified that aspect in his feedback to the post.
Regarding the vegetarian diet, my fist contact with the teaching has been through the book Kemetic Diet, which I tried to implement right away. This is still the basis of my nutrition but, had to supplement it with fish as tofu and other replacement for meat, did not suit me well. I am still trying to find the proper balance allowing me to avoid utilization of meat totally.
I believe because of your early practices, you have passed some mental barriers as they relate to regular practice and study; the systematic approach of Shetaut Neter will certainly provide you with good opportunity for further authentic spiritual growth.