About the edge streaking, the solution is to process the stack starting with whichever end has the narrowest field. You're apparently shooting in the opposite order, so just do a File > Reverse Order before doing the Align & Stack All.
The resulting image will look like the central portion of what you're showing here. It's almost equivalent to cropping.
The standard question after that is something like "But isn't there some way to start with the widest frame and get clean edges even though they are not covered by all the source frames?" The answer is that there is not, in the current version of Zerene Stacker. Other products have the same issue, with various ways of handling it. What Helicon Focus does is to figure out for itself which end of the stack has the narrowest field, and it automatically starts at that end. This approach provides images that are free of edge artifacts, which is nice behavior especially for new users because it avoids drawing attention to the edges where strange things happen.
About the "clouding in the back", I can't tell for sure what is happening.
One possibility is that it's an optical effect in which light from out-of-focus foreground regions is getting added to dark parts of in-focus background regions. This happens when you're focused on the background and the entrance cone of the lens happens to be wide enough that it runs across some bright foreground. Essentially, the bright foreground is "hiding" the dark background from part of the aperture, so that chunk of dark background looks brighter than it should. If that is what's going on, there's not much you can do about it except to stop down a little more. The narrower you make the entrance cone, the less vulnerable it is to seeing spurious foreground. Of course you have to trade this off against decreased sharpness due to diffraction.
Another possibility is that you're picking up some flare, perhaps due to the large bright background area. Some lenses benefit a lot from a simple added macro lens shade. See for example the first panel of photos
HERE.
In either case, you can resolve whether it's happening in the optics or in the stacking by looking at your original source frames. If the source frames are clouded too, then it's happening in the optics. If the source frames are clear, then the problem must be introduced in the stacking. If that happens, I'll be very interested to explore it further, since in several hundred stacks I've never seen this sort of clouding effect get introduced as a stacking artifact.
I'm sorry to hear that the 1.4 extender is not working well. Sometimes they do, sometimes they don't.
--Rik
Edit: to expand the discussion.