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The lengths get folded in half, so it doesn't matter which one is which. The reason for pre-cutting them to different lengths is to save cord, since the strands get dropped anyway later on it's better to start with them about the right length.
12 inches is the minimum. I used a bit more (about 18 inches), how long it is depends on how you want the weight distribution. More weight makes getting a loud crack easier but means the whip is slower to use and heavier. Too little and getting a crack will require very fast swings.You can stop wrapping the core wherever you please, frankly. The artificial sinew is just to bind the strands together enough that it's easy to plait the belly over the core. It's really annoying to manage all the strands without it, though it can be done.
That's the "spanish ring knot" style expansion or a doubling. I prefer to double behind, it makes the knot lie flatter, but doubling in front makes the bights stick up a bit, and looks messy.
Sorry I didn't get back to you sooner, glad you figured it out anyway.
Looks excellent, glad it was enjoyable!
Sorry for not replying sooner. The groove is pretty much just to make it a bit easier to tie everything on, as you discovered it's not needed to get a functional whip but saves a bunch of time.
The keeper starts out much like the overlay, just with fewer strands in the flat plait.
Not really. The only thing that truly matters is that the whip tapers smoothly and is reasonably flexible. With paracord a single belly around a core will typically work well for a reasonably sized whip, but with thinner cord you'd want a few more bellies to get it large enough to be comfortable to hold and to get the stiffness/flexibility right.
Turk's head engagement ring