They decided to follow through with the plan the moment everyone memorised it. Like planned before Lia would be on her own but there will be enough corpses for her to use her power on. And as no one knew of Lia's identity the surprise element was their advantage.
The other upside was that at the moment, there were no vengeful ghosts roaming around they had to take care of because Min all sent them to the Mortal World. So it was one less worry.
If Xavi and the others successfully managed to keep the private army busy and Lia managed to sneak in, they could probably aid her but at the moment, it didn't look like they could gain the upper hand.
There were five captains in the private army but they all had their own squad and soldiers to control, while Xaxi, Mika, Susa, Nana and Noel had to be on their own fighting against them.
