Thank you Kurt, John, David --- this time/effort is really appreciated!
Now I understand the nomenclature for negative delay is due to definitions in VNWA application environment, different from HP 8753 etc. I thought it was something about definitions in the software, but this explains it very well.
Yes, 3.33... pSec/mm (I first wrote ~3psec/mm) but of course that was just lazy by me. The Vf discussion is very helpful, and this tells me that deriving delay solely from mechanical dimensions is not likely practical where the BNC adapters and connectors have use of dielectric material (this is also true of the APC-7/BNC adapters I mentioned early on - they look like half of a BNC in-series adapter grafted onto an APC-7 connector, so at some point inside there is a transition between dielectric material and the air dielectric plus bead in APC-7.) The HP 8753 appears to know only the 3.333 pSec/mm rendering of length and time as far as I've seen so far (and HP seem to specify only "insertion length" when I look at their various adapters which have that data). More complications!
HP 909A, 909C, 909F, 85050B - clears up my confusion on these parts and tools (and I now see some offerings of 909F load as "6 GHz", and a different part number as broadband APC-7 load in the 85050B kit (85050-60006/85050-60021). It was good to learn about mechanical "collet depressor" feature in 85050B open standard! Alas, my existing 909 is only a lowly "A" type. Perhaps I will find an 85050B at tolerable cost; I do not think I can make use of it's sliding load or cal standard data tape as I do not have HP 8510 series analyzer (except in dreams!)
The 4-pc BNC cal kit is now on order from sdr-kits. John's experiments and explanations will be very helpful once the cal kit is in hand! It will be interesting to compare these components (which have data) with the components I currently have (the Fairview Microwave loads, the BNC male/female shorting caps that have now arrived, and the UG914/491 parts also here). John's documented efforts in this thread will illuminate a path forward on that front. The APC/BNC adapters remain a curiosity (I hope at some point someone reads this thread, sees the photos I attached, and can identify them!)
This has been so far, and hopefully continues to be extremely helpful and great learning opportunity - much thanks to (John, Kurt, Dr. Kirkby) as well as VNWA group.io moderator who has been patient with this long discussion!