Alright, lot of information to sort through
. I don't have an M&P but I am an avid 45acp reloader so I'll try to help out.
If you are having a hard time finding patterns to address testing several reloads through one gun it might be because there are so many variables at play here. Shooting a reload that works fine through a new magazine is even adding another variable.
At what point are they failing to feed? Does the nose bury itself into the feed ramp or is it getting caught at the top of the chamber before the rim makes it under the extractor? Or something else?
My dummy rounds (no powder/primer) that I make when developing a new recipe all fed properly -- even when I was intentionally guiding the slide. This was tested after I disassembled and cleaned the firearm.
Is this a new load that you worked up for the M&P or are these from the work ups you did before that FTF at the range? Also intentionally slowing down the slide is a good test to check a too tight extractor on a 1911 but doesn't replicate the true feeding of a gun.
I know every handloader is different and has different goal in mind but 8 loads, all target, for one caliber seems excessive. The powder inside isn't going to effect feeding but the external dimensions (and the Lee FCD) will. Perhaps in would be easier to manage two guns with different tastes by finding the one load that works for both and stick with it. From reading what you wrote it looks like you have as many as 3 already.
Also mark your magazines with numbered masking tape on the bottom and record jams by magazine as well. There could be more than one problem at work.