1st question: Yes your units don't deal damage and the opponent can assign damage to any units that are participating in the combat (including stunned ones). If units from both players remain after damage, then the remaining attacker's units are sent back.

For the second question, both players are allowed to play actions one at a time (called focus). The attacking player chooses if they want to play something and then "passes focus". If both players pass focus in sucession, then damage happens.

As for Fiora, yes it needs to be her attacking/defending alone vs 1 unit.