It only jumps 6 degrees with your vacuum advance at 12 - 15 inHg? And you only have 8 degrees initial? I would get a different vacuum can if I were you. How much timing do you have a 4000 RPM with the vacuum advance disconnected? Vacuum leaks can definitely cause those symptoms too, I hope you find the problem.
I think the engine wants more timing than that, however, dieseling is usually caused by excess heat in the engine, and/or too high of RPM when you shut it off. ALso, if more air is available (vacuum leak), it can pull in more air even though the throttle is closed, which makes it easy to diesel too. Also, I have heard dieseling can be caused by too much timing advance, but find it hard to believe because when you shut the key off there is no spark, it's dieseling from cylinder pressure, or heat at that point, not timing. I could not see the video.
BTW, that peice of fiber/plastic from your 4L80E is the front selective washer that goes behind the pump to set front endplay. It probably got too hot, got brittle and broke. If the external trans filter was cold when the trans was hot, that means it didn't have any flow, which means your cooler or external filter got plugged or blocked, and the tranny was not cooling or lubricating. So it got ruined from heat most likely, which is the most common cause of auto tranny failure.
Also, the reason you have never had a problem with FRAM filters is because you never know there is a problem unless you block off your filter bypass. When one of the filters goes bad and plugs itself up with the crappy fibers on a typical engine, the oil filter bypass opens up and everything continues as normal, it's just filtering NONE of your oil.
On race engines, you can't use the crappy fram filters because they plug up, and with the filter bypass plugged for 100% oil filtering (no filter bypass allowed), you lose oil pressure with a FRAM because the filter blocks flow, and then the pressure relief spring in the pump just dumps all the oil right back into the pan, causing zero oil pressure through the engine. This has happened on brand new FRAM filters, they are not good.
Very nice build by the way, nice engine, nice rust free rig, good job building everything. I like it.