#1. This is intentionally designed to do exactly what you see. If an image is placed under a specific heading, it should not bleed into the next heading. There are other reasons for this, as there are other kinds of floated content in VaultWiki. It also becomes an issue again with #3.
If you really want a specific image to bleed into the next heading, then you have to break the isolation between the headings. You can do this really easily by wrapping a heading in a DIV tag. This will remove all controls from that heading as well, which prevents unexpected behaviors when the heading isn't isolated (such as editing text from a different section rather than the one you want to edit).
#2. I find it really odd that overflow: hidden actually works that way. I would have expected the control icons to become hidden, since they are part of the overflow. I will keep this in mind if it's needed in the future, but with the described use case it's really not needed, since the sections are supposed to be float-containers anyway.
#3. If you add clear: left and clear: right to those respective classes, then you cannot have multiple floated images side-by-side. The clear properties were intentionally left out to allow for this case. Also, from what I understand, changing it will break the GALLERY tag.
Not only that, but we found over the years that it was more desirable to have an image be near the text that it illustrated, rather than let other images above it keep pushing it down.
If you have a problem like this with overlapping images WITHIN a section and you do want the images on top of each other, try placing the images in a GALLERY tag and setting perrow="1". I've gone ahead and added an align attribute to the GALLERY tag for the next release so that you can float it without invoking a DIV tag around it.