So I understand that Kaalia of the vast does not allow attack triggers. However, say I drop in Aurelia, the warleader with kaalia then use relentless assault to take an extra combat will Aurelia trigger in that combat. It is the first time she has been declared as attacking but not technically her first attack.
508.4. If a creature is put onto the battlefield attacking, its controller chooses which defending player or which planeswalker a defending player controls it’s attacking as it enters the battlefield (unless the effect that put it onto the battlefield specifies what it’s attacking). Such creatures are “attacking” but, for the purposes of trigger events and effects, they never “attacked.”
In short, no. Using only those three cards, Relentless Assault would not untap Aurelia, who entered the battlefield tapped because of Kaalia. If you find some way to untap Aurelia, then yes, that would work.