Does Ahri legend affect an ambushed Inferna, and can Blue Ahri decline Deflect cost?

The Inferna that ambushes in is affected by Ahri legend, but not by Blue Ahri's ability, because that ability already chose its target before the ambushed Inferna was at the battlefield. Ahri legend triggers for every unit designated as an attacker for the first time in a combat at a battlefield you control. Any time a trigger incurs a cost like Deflect, you may decline to pay it, in which case you stop playing that trigger and it has no effect (rule 404.2).