New Adjustable Grip Hitch ?
Some years ago I was looking for an universal adjustable grip hitch. I liked the Chisnall as a good universal one, but not secure enough to replace e.g. the Blake’s Hitch or Icicle Hitch.
This (new) Hitch is based on both my favorites: The general purpose Ground Line Hitch and the Chisnall.
It is basically a Ground Line Hitch with two or three additional turns preceding it.
The advantages of this hitch are: Easy made, easy remembered, easy untied, firm, secure (better than Chisnall), jams in both directions and holds also in slippery rope or line.
I use it instead of the e.g. Rolling Hitch, Blake’s Hitch, Adjustable Grip Hitch, Icicle Hitch and even the Prusik Hitch!
I still cannot believe that this quite simple hitch has not been invented/ described earlier!
Welcome, Zigzagger! 
Your post is too long devoid some comment --sorry!
I tried your knot in some handy fibrillated polypropylene
5/8"(?) laid rope, and it fared poorly; but then so too did
the Prohgrip / Blake’s hitch (really surprising, and
even w/some slight adjustment of adding a wrap), and … .
Finally, I got something that seems to hold, for making
an “adjustable” eye (though I might care more for having
great strength vs. adjustability --methinks that hitching
to the line like this should be very strong, as the SPart
is broadly gripped and not much deformed).
I like that you like the ground-line hitch and sought to
adapt it into other service. Btw, do you know that this
hitch is much used in commercial fishing as a sort of binder
knot, tied in reverse on thin line (also twin lines) that
spiral wraps say two headropes or the grouped-together
edge of netting?! And in this use, the back & forth hitching
is often repeated beyond the just-once state of the GLH.
The knot that I found holding while others slid (noted above)
is something I think of as “half-Hedden”, as the finishing,
traced wrapping of the tail just reaches the far end and
stops, not further following the first part back out --the
Hedden hitch is typically tied with a sling, so has the
“twinning” built in via that --wrapping with a bight, and
only tucking the SPart out through this bight’s tip.
(I think I’ve got a half-turn more than the Hedden.)
Cheers,
–dl*