Can Janna heal before combat damage, and do units played during showdowns enter exhausted?

You cannot play Janna as a reaction to heal once combat damage is being assigned — both players have already passed focus by then. Janna's healing is for marked damage from earlier effects before combat starts, or to heal and push a unit during a showdown outside that damage-assignment step. Unless specified otherwise, all units enter play exhausted regardless of when they are played, including Reaction and Ambush units during showdowns.