it fixed itself?!
So after the original post, i pretty much gave up on the idea of having 2 hdmi monitors coming out of my dock. Several days have passed and literally nothing has changed to my knowledge.
I connected the dock as usual this morning, so i could use the hdmi 4k adapter, and i noticed that the audio options were listing a new output device: "Digital Output (S/PDIF) - Plugable UD-ULTCDL" & "Analogue Output.." -- this has happened once before, i'm sure. So i connected a second spare monitor from my office to the hdmi 2k adapter and it worked.
Voila, dual screen again...
I've attached another round of support tool output for anyone who has the knowledge or patience to cross reference them - i would love to know _what_ made the difference!
TLDR; something clicked today, as it started working without any new input from me. see file attached.
¯\_(ツ)_/¯
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