A supporting UG in melee has the Shove characteristic.
I assume if its die rolls S or Skull it will trigger the immediate bonus white die roll even though not frontally engaged- I don't see anything to say it wouldn't, but I am not sure if this is the intent, or how it is being played. (One of the downsides of only playing solo so far!).
And if so...
Is it possible to get multiple bonus white die rolls against a single UG, from a single combat? (e.g. up to 3 bonus dice, if overlapping on both sides). Again I see nothing in the rules to say no to this - but in Maximus/Magna only a single shove effect occurs per file combat so it feels like there should only be one bonus white die per file combat for equivalence?