Damn, I thought I had solved this... Still, a lot of the logic I used applies to this new situation as well.
1. The Master of Masters wants to restore light after it expired. He has created some data worlds and trapped the Dandelions in them. The natural connection for these two things is that he is creating a utopia dominated by light. He locks the device that runs the data worlds into a box, transports it to the far future, and then plans to turn the data worlds into real ones, restoring light. If this is the case, he would want the Dandelion leaders to be as hard as possible to corrupt by darkness. He seems to have used humility as the key characteristic, as can be seen by observing how they react to being selected as leaders. Also, humility is the opposite of pride, the missing deadly sin. Now, since Lauriam is not humble and also ended up becoming a villain, it is clear that he isn't supposed to be one of the leaders. What is wrong here? MoM knows that this simple version of the plan will not work, so he has to adapt it to deal with the problems that have arisen. Lauriam being there must be a consequence of such adaptation.
2. Ven was named as replacement by Ava. If he wasn't, he wouldn't know where to go to meet the others. Also, he would have to deal with a very disapproving Chirithy, who probably would have ratted him out a long time ago. Additionally, Ven told the others what Ava had said to him, and Ephemer & Skuld confirmed this by saying that they got exactly same speech.
3. But Ava was very reluctant to make any changes. Even when she gave the Book of Prophecies to Brain, she also gave him the list of names, so he could see the change and revert it if he thinks it is the right thing to do. So, if she did name a replacement, she would have informed the others about it. Except, if it happened under circumstances where she had no possibility to do so. The battlefield of the Keyblade War provides such circumstances. With Lauriam, I assumed she was mortally wounded when she did it. With Ven, she probably just wanted to save him from being killed in the war.
4. Lauriam's behavior as union leader seems out-of-character to me. He is bending the rules to avoid conflict between the Dandelions, such as delaying re-establishment of the five unions, and shifting Shift Pride from a real combat sport to a video game. Other than that, he isn't doing any leading at all. Yes, he is genuinely worried about Strelitzia, but it seems to be a bit too much for him to neglect his duties because of that. It is also exactly what Ava would want him to do if he was the replacement, which is why I suspected him to be in the heart hotel business.
5. Just to pre-emptively counter any nonsensical speculation about Ven's darkness that is going to follow from this: Xehanort extracted all darkness from Ven's heart when Vanitas was created. Any darkness that is there in KH3 must have gotten there after that event, and therefore cannot be conneted to what happens in KHUx.
So, what do you think? Have I missed something?
Edit: Also, it seems to be proven that the Darkness is Not A Very Dark Darkness. All it is doing is helping to get rid of Maleficent by sending her back to her own time. In fact, it probably is MoM himself stopping by...
Spoiler
