Ashes Reborn: Rise of the Phoenixborn

Yes — a Phoenixborn can be declared as the single guard for a targeted unit even if it is exhausted (page 9).

You resolve each attacking unit one at a time in an order you choose; an unblocked attacker deals its attack value to the Phoenixborn and then receives 1 exhaustion token, while blocked attackers follow the counter/block resolution before exhaustion is placed (page 6).

If the target unit does not or cannot counter (for example because it's exhausted), total the attacking units' attack values and deal that amount of damage to the target unit, then place 1 exhaustion token on each attacking unit; the target unit does not become exhausted if it didn't counter (page 9).

When a unit counters, it and the attacking units deal damage as a battle, and if the countering unit is assigning damage among multiple attackers the defending player chooses how to split its damage among those attacking units (see the 'countering' example on page 8 and rules on page 9).

When you play a copy of a ready spell onto a spellboard that already has that spell, the new copy is placed beneath the current copy and does not take an additional spellboard slot; Focus 1 becomes active when you have two copies (Focus 2 when you have three), enabling the listed Focus effects (page 11).

Wound tokens are tracked separately and do not change a card's life value; when a unit's or Phoenixborn's wound tokens are equal to or greater than its life it is destroyed, and some effects can place wound tokens directly without dealing damage (page 12).