The clear implication was that Londo presented this (to all concerned) as a last-minute operation. There's no good indication that this was planned very far in advance, not least because Londo only seems to have decided to kill Refa in response to his arrival on the station and the presentation of a threat to Londo's position.
JMS spoke to this on the Midwinter site.
Q. Why didn't Refa's allies help him?
JMS: Bear in mind that all of Refa's people were back on Centauri Prime. He didn't bring them with him; to send word to Centauri Prime to bring them all the way out here, then on to Narn, would double the time required to get there, and by then the "rescue" would've been over. That was part of Londo's scheme...he wouldn't have much time, he had to get in and get out. Refa only brought a few with him, and they were needed to watch Londo and guard Vir.
Midwinter.com - And The Rock Cried Out, No Hiding Place
and
Q. [How did he get in touch with G'Kar?]
JMS: I think that if Londo sent word via the Babcom unit that he wanted a one-to-one with G'Kar, and that thousands of Narn lives were
on the line, he would come. G'Kar has nothing to fear from Londo as
long as he's on B5...and if anything, I think he'd be amused if Londo
did try anything.
Midwinter.com - And The Rock Cried Out, No Hiding Place