OK, good, my adjustment appears to not be a fluke. I just got to L14 on the machine.
For the record, this was the critical adjustment: I'm really, really pushing it on the leftmost key. Rather than going for the center keys, which seems to be the standard pattern, I'm taking advantage of those first few seconds of "low traffic" by jumping straight up and making a mad dash to get that left key up as high as possible until it's either in place or the snappers have forced me to retreat (whichever comes first). The goal is to push it at least into the "top flight path" region, which allows me to quickly swing over and pop it in there when the opportunity presents itself, preferably after placing the two left-center-keys.
I haven't played the regular version on MAME in a while. I don't know what it is, but it just seems like the physics of a joystick lends itself better to Junior, since so much of the game is about moving into and out of precise little "notches." The keyboard is too fluid and sensitive. Junior is in the wrist, not in the fingers! For me anyway.
The truth is, I think going back and forth between the two versions, while it accelerated my learning in the beginning, is now hampering my technique, because the approach you have to take on the chains is so different. You can move laterally along the safe spot on the regular version, which I virtually never do and have never gotten good at because I'm not used to doing it that way. I'm constantly zipping up and down the chains needlessly because I'm trained to have to do it like that. In other words, I probably would have killscreened the regular version by now if I weren't slowing myself down trying to master two different ways of playing. Or maybe I wouldn't have, who knows. We can't all be Ben!
I'm not in a hurry though so it makes no difference to me when it happens. In fact, killscreening one of these games is always a little bittersweet, for the same reason that reaching any goal is.