As the esteemed Barrow Wight said and also, the perceived
Diplomatic Immunity of the heralds only came into being much later (Tudor/Elizabethan and afterwards) when their positions became more or less those of petty officials who marshalled tournaments or 'talked up' their lord's prowess in the joust, rather than the battle messengers of earlier times.
However, Bethberry's notion of Elrond as herald and Arwen's banner making, is something that I had not considered before...One of the reasons why I love this place