That relationship between Ilisidi, Tabini and Bren is very interesting. One thing running around in my mind is why Bren is sent to the government center in Koperna, where he is supposedly acting as an official, not just paidhi. Why on earth would Ilisidi put Bren in front when she knows Marid people have a very negative view of humans?  We have Paigiti's insulting words as an example. 

I remember when Bren returned from the Kyo encounter and Tabini sends him to Mospheira because the Mospheirans need to know the true value of the paidhi-aiji. And in line with that Bren wins over most of the humans he deals with (except for some diehard Human Heritage party folks).

So I think is Ilisidi doing the same thing here? He is not sent with Bregani just because Banichi is in charge of city security, but because she knows Bren has the capacity  to win over all sorts of people. After all Cajeiri tells his parents that lord Bren can talk anyone into anything. And Bren charms just about everyone. He steps in to heal wounds even when no one asks him to do that. He is like a roll of sticky tape winding around all sorts of people and gluing them together.

I'm not sure I want to insist that this is what Ilisidi is thinking, but it does occur to me. And I don't see any particular reason against this interpretation.

A few points. I go back to a point I made a long time ago. The Aji and the Dowager more or less share power as co-Aiji. Ilisidi of the east and Tabini of the West. TWO regions with TWO Aiji. Two is very unstable. Two cannot be allowed to stand. So Bren is in effect a third (much, much weaker) leader. He has proven himself to be useful over and over again. He is a peacemaker. And he cannot possibly take power for himself. Everyone including him knows it. He is safe to give power to. Also I think that the intention is to eventually have three regions, West, East and Marid. 

I think that the process of making a new lord is a three step process. Step 1 is that someone nominates a lord. Step two is that Tabini accepts the nomination. Step 3 may or may not be official. A new lord must have the support of the people, their own and in general. Even if Tabini accepts the new lord, if a powerful lord like Ilisidi or Tatiseigi opposes it, the nomination could fail. Since Tatiseigi was the one who nominated Nomari and Tabini believed that Ilisidi would support the nomination, then Nomari was confirmed. But Ilisidi then told Tabini that she wanted to have input in the situation so the official approval was held up. 

I believe that Ilisidi is very old and ill. A few books back she almost passed out and dropped a glass and Bren and Cenedi had to practically force her to go to bed. I believe that Ilisidi has been having more of these incidents lately. She needs her sleep more than before. She can't go night after night without sleep anymore. She wanted Bren there as her backup. He can do things she cannot do anymore. She cannot physically stay awake for 24 hours straight setting things up. At one point Cenedi had to ask Bren for permission because Ilisidi was asleep and he didn't want to wake her. He knew the her plans but he couldn't give orders on his own and he didn't want to wake Ilisidi because (I believe) she had another of those fainting incidents.  (I think she's setting up Bren to be her successor for her affairs in the west. She'll name some young relative we've never met as her heir with 3 regents to rule until they reach adulthood, Bren, Cajeri and her cousin. Her cousin handles the east and Cajeri and Bren the west.)

Also, Ilisidi didn't understand the inner workings of the Marid much better than Bren does. She could easily make a major mistake. She was walking on very shaky ground she didn't know or understand. She was relying on Machigi for her information and Machigi is a very uncertain ally at best. Machigi could be setting her up for a double cross. So she sent Bren in. He's a peacemaker by nature and being human no one expects him to completely understand things. He can make a minor mistake and it won't wreck everything. She can disavow any mistakes he makes and fix them. It won't hurt him politically in the west because he's human and no one outside of the Marid really understands the Marid anyway. 

Look at Bren and Topari. Topari called him a creature in one of the early books. But when the Dowager visits his capital he turns to Bren for advise. He trusts Bren to be honest with him. Bren can do that with people. People trust him and he has a reputation of being trustworthy. He never tells people to do things which will ruin them. Sending Bren in to negotiate throws everyone off balance. He is known to be the right hand man of both Ilisidi and Tabini. He has unofficial power in his own right as a respected lord. He has a reputation of being unpredictable but honest. He shows his emotions on his face way too much and people believe they can read him. But he has never been know to lie while negotiating. He's a peacemaker and a total wildcard. The people of the Marid will pause and think before doing anything because they don't really know what's going on. This pause helps Ilisidi and her allies. I think that Ilisidi sent Bren because of the fact that the Marid has a negative view of humans, not in spite of the fact. They will be thrown off balance and Bren can then act as the calming influence in a chaotic situation.