3. Adonalsium - honestly it is quite simple. There is mention that it didn't fight it's splintering at the end. It also had the prescience of knowing it will be splintered. It makes sense that the sum is greater than the parts and it's abilities vastly outstrip even Cultivations's. We have now the shard of retribution - what is the biggest crime in the cosmere that we know of? The splintering. Why did Adonalsium allowed it's splintering - if you are THE CREATOR, you have to move away for some time to allow the creation to develop. The opposite is way too boring - the temptation to intervene and the capacity to shape will be too great. Braize is a planet that attract souls. It will eventually attract all the Vessels - it already attracted 3. Where Retribution awaits for them. TOdium don't mind absorbing new shards. So they will come, be absorbed and of course the last one absorbed will be Harmony that will bring back balance to the force. Then Adonalsium be restored enriched with the experience of all the vessels, shards, spren and every other invested thing. Couple of thousands/millions/trillions of years later it will either repeat the splitting trick or dump a shitload of investiture to keep the universe going, open the door to Beyond, go trough it and start the cycle anew by creating a new cosmere. So Braize is just the plan for reassembly.
1. Yeah, you can't just drop such a bomb as a passing remark and everyone just shrugs.