Matthew_, your first post. Welcome to the forum.
Did you mean stretch instead of sag. Sag is determined by the angle your suspension comes off the tree/support. Long ago, ENO's starps (I don't know about "Slap Straps" were made of nylon. Nylon stretches - so that can influence sag over time - as it elongates. Eventually, ENO saved up enough money to buy a clue, and their hammock straps are now made of polyester - like all the other vendors.
I don't know what an AT knot is, but the two simplest setups use daisy chain or webbing (i.e. cinch buckles).
I don't know what a speed hook is, but here's the reason to use a dutch clip, soft-shackle, or carabiner. Imagine without such an "opening" device, you have to slide one end of your strap through the loop at the other end and snug it tight against the tree. That's fine if you get your height right the first time. But usually some adjustment is necessary. There's a law of nature that dictates if you need to move your strap up or down on the tree trunk (get a better angle, set the hammock the right distance off the ground, avoid a mess of tree sap) there will be some branch in the way. Without a link (dutch clip, soft shackler, etc.) that opens, you'll need to disconnect the hammock, pull the suspension strap out of the loop end around the tree, reposition the strap, rethread it, and reconnect it. With a soft-shackle or carabiner, you just unclip it, move the strap, and clip it back.
Note: if you use a carabiner, make sure your stresses are along the strong direction of the carabiner and NOT creating a force on the gate bar, perpendicular to the joint.
YouTube is your friend. It takes a lot of words to explain something you can see in a few seconds of video. I think someone made an often quoted statement about that.
The ridgeline holds the ends of the hammock at a fixed distance. So as long as your trees are a further distance apart than that "end distance" - and they will be because your tarp is longer than your hung hammock, the ridgeline keeps the ends a fixed distance apart (your ideal sag) if you have a less than optimal hang angle. Essentially, it transfers the forces from the tree/suspension/hammock/suspension/tree to tree/suspension/ridgeline/suspension/tree, and the hammock hangs as happy as a lark with the ridgeline taking all the work.
The hammock isn't hanging "on the ridgeline" exactly. it's just that the ridgeline connects to the ends of the hammock at the same place as the suspension. The ridgeline is made the length you want the ends of your hammock to be when hung. So you set up your suspension such that the ridgeline is just snug.
