Learn English – Correct use of bound/bounded

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.

Here is what Bryan Garner, Garner's Modern American Usage, second edition (2003) says about anymore used in the sense of "nowadays":

anymore. A. Meaning "now." In the sense "now," "nowadays," or "still," the word anymore fits in three contexts: (1) negative declaratives {you don't bring me flowers anymore}, (2) yes–no questions {Do you go there anymore?}, and (3) hypothetical clauses introduced by whether or if {I wonder whether they go there anymore}. In sense 1, the meaning is "now" or "nowadays"; in senses 2 and 3, the meaning is "still." When anymore is used in some other type of positive statement (not in sense 2 or 3), it is dialectal—e.g., "The price of housing is outrageous anymore {read these days or nowadays}." In a [1986] linguistic study of Missourians, informants considered this dialectal usage "well established though controversial." [Citation omitted.] That mans that the informants were all familiar with it, but many didn't like it. The findings would probably hold throughout most of the United States.

Garner notes that anymore is standard in U.S. English when it appears in negative constructions ("Don't do X anymore"), in questions ("are they X anymore?"), and in hypotheticals ("whether we X anymore"). But in positive declarative statements such as "People are rude anymore," Garner holds (accurately, in my opinion) that the usage is not standard across U.S. English, but dialectal.

The form of expression has been around for a long time, as is evident from these examples reported in Harold Wentworth, American Dialect Dictionary (1944):

1903 s.e.Penn[sylvania] Lancaster Co[unty] There's just only this one any more. Martin 'Ellie's Furnishing.' ... 1929 Iowa 'Any more I don't like the boughten [ice cream].' Characteristic. K. Buxbaum. 1930s w.cent.W[est] V[irginia] Charleston We still use that custom anymore. 1931 s.w.Penn[sylvania] It's quite warm anymore..It's pretty poor anymore..Anymore I never see him. ... 1932 cent.Ill[inois] Dewitt Co[unty] 'We used to go to Weldon Springs for picnics, but any more we go to Salt Creek.'Common. C. W. Carter, Jr. ...

Although Wentworth indicates that Northern Appalachia (West Virginia, Pennsylvania, Maryland, and Kentucky) is the primary locus of such usage, he also finds instances from New York, Ohio, South Carolina, Kansas, southern Ontario, Michigan, Montana, and (as noted above) Iowa and Illinois. The examples of dialectal any more in American Dialect Dictionary, despite being set in small type, come close to filling two complete print pages. Still, notwithstanding such wide distribution, the expression remains dialectal—and Wentworth reports no instances from New England, the Pacific states, much of the interior West, the Southwest, and much of the South.

So the answer to your question "is the positive use of anymore considered correct spoken/standard written English?" seems to be that it may be considered so by English speakers for whom the dialectal use is natural, but not by others. And as Garner remarks, even in areas (such as Missouri) where the usage is currently familiar to many people, "many didn't like it."