Lex sent Paul and Mei to a different room so that the two of them could talk amongst themselves privately, and figure out what they wanted to do. Lex did not think that Mei would leave her army behind, or abandon the Origin realm, in which case there was nothing he could do to help her.
The reason he kept wearing a mask was to hide his identity so that no one could associate his actions to the Midnight Inn. That was why he did not use the Masked Tyrant technique out right, for that mask was too identifiable since it made Lex look like a devil!
There was one more reason, though. Just as Velma was currently in the Artica race with the Humanoid Alliance, he had one member of the Midnight Inn inside the Origin realm, carrying out a long-term mission.
