Do impact and phalanx casualties get counted in the melee result?
If the enemy charges me we fight a round. If I want to 'keep fighting' I charge them on my turn, and we both only hit on 6s. If they want to 'keep fighting' they repeat next turn and we're back to regular attacks. Correct? Further to this, I assume the option to not attack back exists so that an enemy can't "use up" my elite unit's attacks by charging in a weak throwaway and then following up with their own elite?
Is there any way for a hero in a unit to be singled out? Are spells like drain or melee attacks or 'sniper' meant for this, or are heroes only hit when they stand alone or after their unit is wiped? If a hero is being singled out, do they still use the lower defense value of the majority models of the rest of their unit?
For heros in a unit, does the whole unit need fast to be fast? Does any model with slow hinder the whole unit? Is there any reason a hero riding a dragon or ice phoenix can't join a unit of chaff?
Your next paragraph is correct as well. Makes sense you put 'keep fighting' in quotes, because in this game each combat is separate. This means that the first charging unit had to back up enough to let the other move away or shoot or charge back. There is still the one-full-quality-melee-per-turn restriction, so you are right that they will hit on 6s. But the second unit still gets the benefit of charging back.
I'm not sure what you mean about melee attacks singling out heroes. I suppose if the hero was isolated enough that much of the unit is not in the fight you could more easily affect the hero, but you are playing Regiments, and that might be harder to do. I've wondered if it would be interesting for heroes to be able to choose targets, so heroes might fight against each other. If the banner rule was a specific figure, instead of a trait of the unit, you could have the hero knock down the banner, which would be thematic. But I suspect balance would be thrown.
A Hero with Fast or Flying could join such a unit, but the other figures would not gain the benefit. I think it would just affect how you moved that particular figure, with the regular spacing still being kept. Similarly, I think a hero without Fast that joined a Fast unit might fall behind in the unit, or might slow the unit down so as not to be left behind.