I have sorta' dropped out of this thread but I will say this......Underground leaks are possible but they SELDOM exist. Every effort should be made to discover the leak above the surface which is where it is maybe 98% (or more) of the time.
For sure duraleigh. I feel I have got to that point as I have spent a lot of time testing and troubleshooting everything I can above ground. I am planning to do a bit more testing above ground this weekend before I get out the shovel and power drill again.
Something I didn't share...when I dug to find the T joint where my suction line goes out to each side of the pool, I had a huge clue for where to dig...the decking boards in a couple areas had newer stainless steel screws...so someone dug here before I did and perhaps fixed the problem in that area...but from what I have seen and shown in this post, wouldn't most agree based on finding metal worm clamps underground it would lead one to believe there surely may be an air leak underground?
Also, the video I posted...it sounds like air is in the suction line at that point...which means air is getting in either at that point, or further up the lines toward the pool/skimmers, correct? Meaning there has a be an underground leak, right?