Same here, backing off the grease fixed my squeaks too.

Three or four plates on one run isn't even that wild in production builds, I've seen whole upstairs floors on a single feed in Buckeye and Goodyear. The trick is you have to do it with the last jack as the end of the line, not tap off every plate like it's a water pipe. Half these crews are piece work subs who get paid by the run, so one long chain is faster and cheaper for them, and the bad connector behind the painted-over plate is just what happens when nobody checks it after drywall. On the new builds I've worked, chained outlets are normal, the real problem is they don't leave a map or label anything.

Check the plank maker's spec before you trust the pad that comes glued on. Some of those pre attached pads are super thin and only there to pass a basic sound test, they are not meant to be your only layer. But stacking a second foam pad on top of that is where the popping comes from, because now the plank floats on two squishy layers that move at different rates and the click edges work against each other. If you want more sound control upstairs, go with one solid underlayment rated for the setup or glue down the planks instead of doubling up. The Cedar Rapids guy was right about the flex, he just skipped the part where the answer depends on what the plank already has.