2.) "Foreground" option
3.) "Section" info
4.) "Circle Cursor" tool ()
It does basically the same as Rect Cursor. You can only really notice a difference placing a HUGE circle. In my opinion, this function is completely irrelevant.
5.) "Line Cursor" tool ()
Rect Cursor already helps you to place a rect line of objects. It is also extremely rare that you want to do a long, rect diagonal line, I can't think of common level designs that requires that. But in any case, you can place a diagonal line of objects manually, and it's not worth a tool for it.
6.) "Fill" tool ()
This tool is extremely risky to use, a single mistake and your level is ruined. If you wrong-filled an area you can't undo it since the game doesn't support undo functions (yet). I also don't think it is really needed since you can already fill blocks fast and safely with Rect Cursor tool (which SMBX 1.3 lacks of, and probably the reason "Fill" tool is actually useful there). But I do think the removal of this function is much less relevant than what I mentioned above.