He'd definitely get to perform an attack, just not both. He CAN choose to attack an out of arc target instead of his retaliation. SAR says perform an attack, does not mention opportunity to attack.
I think you need to read my example and the Simultaneous Attack Rule again. An "opportunity to attack" is a key phrase in the Simultaneous Attack Rule, and I'm not saying Dengar is denied his opportunity to attack. He still gets that. But in the order in which the scenario unfolded, the Scum player chose to use Kath instead of Dengar's ability first, and because Soontir was destroyed and had already attacked, Soontir was removed immediately after being destroyed by Kath. Keep in mind Soontir was the active ship at the beginning of the PS9 Combat and is not kept in play because of the Simultaneous Attack Rule.
This left Dengar still waiting for an opportunity to attack (via the Simultaneous Attack Rule) but now without a target. Even if the Scum player decided he wanted to use Dengar's ability, Soontir has been removed, and there are still no valid targets.
If by some weird interaction elsewhere on the play area, another ship comes into range of Dengar during the PS9 Combat, then the Simultaneous Attack Rule still being in effect for Dengar would allow him to attack before being removed.
@WWHSD: Kath is attacking before Dengar due to player's choice. Nothing else. I was thinking of a way to eliminate Soontir before Dengar had an attack. Dengar's ability does indeed trigger, but again that's also player's choice as to whether or not to use it. It's quite probable that a player would trigger Dengar's ability and attempt to destroy Soontir first. Maybe the attack from Kath's auxiliary arc looked more likely to destroy the pesky Baron and his annoying Autothrusters. Either way, Dengar is not being denied his opportunity to attack before being removed.
But shouldn't Dengar be removed immediately if he chooses not to use his counterattack on Soontir Fel? He had an opportunity to attack and chose not to use it.