Thanks for the clarifications Aldhick, as always. Good spot on the support advance clarification, although the scenario outlined by Toothpick is slightly different and maybe worth clarification - I've drawn the example to make it clearer.
If unit A, destroys unit 1 (in a round or two)... Unit A can now Advance into fresh combat. Q: Can unit A "Advance" into the combat in front of it, between unit B and unit 2, as this would allow them to support unit B?
I would say not as unit A can not "see" the enemy unit. Therefore unit A's options are fallback or remain.