http://images.**********.net/mangas/bleach/bleach_610/bleach_610_13.jpg?v5
The *********** translation describes it a bit differently to **********. Usually I read *********** as I find it is more accurate.
He sees all the possible futures of what's alive an on his vision of sight. He didn't know Ichibei was going to revive himself or that Ichigo and the others would arrive until he saw Ichigo and saw what had happen. He didn't saw the past but rather his other ability that lets him understand of the things he knows.
It seems speculative whether or not he knew Ichibei would revive. This also seems nitpicky considering what you're telling me essentially means he can basically see into the past and future, just explained in a roundabout and elaborate/sophisticated way. I believe when Yhwach told Ichibei what he sees he also understands he means current/future events. another note is that Yhwach also says "I saw everything", not "I understood everything"
He didn't know Aizen would activate KS, and as Aizen said he only avtivated KS before they arrive in that dimension. That means Yhwach can't see the future of beings in another dimension as we see above he needs to gaze at someone to see the future.
Again, Yhwach knew Aizen was in Soulsociety and went to meet him. This really can't be overlooked.
As for why Yhwach was unaware of the illusion theres a few reasons. The first is that because the ritual for KS was done in the past, it may be harder to detect, and Aizen had to recreate the Future Yhwach had seen with his almighty carefully, to the point where he actually suffered the injuries in the illusions himself (by changing himself into Renji/Ichigo) to not give anything away so Yhwach did not realise that Kyoka Suigetsu was being used.
Because the hand was in another dimension, the same with Uryu he didn't saw Uryu coming before he lost his powers. He should have since KS effects were off.
The hand was in another dimension, yet Yhwach should know about this limitation. The Uryu Arrow thing is probably just PIS,(Heck the arrow itself is PIS) theres no way you can't have PIS/CIS in a story with someone has the power Yhwach has
This are assumptions, after mimihagi hold the SK, Yhwach knew what it was clearly with his eyes.
The only kinda assumption is that Mimihagi can't be predicted, but it's but a pretty good one. And no, he did not. Yhwach sees Mimihagi, mentions the one thing that escapes his eyes and then loses his cool asking if hes the Soul King itself. He would have no reason to do this If his almighty power was working on Mimihagi.
This does not seem like he "understands" everything
All in all i'm not very convinced, I'd like to see what others think.