As kalivon said, there is a 99.9% chance that Morticia will not give a bonus.
On the Japan servers, Cardina was a pullable event unit for the Masquerade arena but in our servers she was a head-start reward in the last event and she does NOT give any bonuses in this event.
And for your other question, an event 6* would be borderline useless to most players in this event.
IIRC, the Dryad event is like the neolilith event and in that event the only thing the event units got was a damage buff.
Unless they tweaked it so that they also give a BT bonus or greatly buffed the difficulty of random encounters and bosses, event units won't help at all.