Yet in this context implies that you expect that the situation will change. In sentences 1 and 2, you don't know, but you expect to find out. In sentence 3, where you leave out the yet, all you are saying is that you don't know.
It is the same with sentence 4. You haven't finished, but you expect that you will. Sentence 5 just tells us you haven't finished. Maybe you will, maybe you won't.
You don't need to have the yet, but the implication is different depending on whether it is there.
Also, for sentence 1, I think it sounds more natural to say
I'm not sure yet if we will have a meeting tomorrow.
This sounds like a meeting that is already scheduled or expected to be scheduled, and you aren't positive if it is taking place. Maybe it is going to snow and no one will come to work.
And sentence 2 has a different meaning than sentence 1.
I'm not sure yet that we would have a meeting tomorrow.
This sounds more like you are uncertain whether tomorrow meetings are something that might happen. As an example of what I mean, let's say I wanted to have a meeting on Thursday but you think that's against company policy, because on Thursdays they lock down the office to sweep for illegal bobbleheads. In this case, you aren't sure if having meetings on Thursday is OK. You'll look into it and get back to me.