[cabmandone]

I've watched that first video several times and my head is still spinning! The duct that runs up the back side and into the crawl space, what size is it?

Where you say in the video it comes from here, to here, to here into the crawl, if that duct running up and into the crawl space isn't sized properly, you can change the flex and you might still have the same problem. It's all about the square inches... and the cfm those square inches can handle. If that drop duct is only good for 1200 cfm, as an example, and you have 24" that will carry 1800 cfm, it's only going to pull 1200 cfm no matter how straight that 24" is.