Along the top line is a "Help" button. Underneath that is a "Help..." menu item. In there are some tutorials. Work your way through them -- they're the best way on getting a handle on the software.
You can't have everything. Where would you put it?
I make sure that none of the object interfering with the shape you wish to fill. If you have an object within the object, I move that out of the way and move it back after fill. I don't know if that is kosher but that was my discovery on this issue.
Gilles is right, that's the problem I found too. I've found using the freehand tool can give you strange results like this, better to use the add points tool instead.
Just use the add point tool (A), then click and drag, it will create a line between two points, then You click and drag from the second point, and so on...
I never use the freehand tool, even with my tablet!
Not everything which doesn't seem to work is a bug. Two points or two lines on top of each other are a common problem in all vector programs. They happen all the time, it can't be avoided, all you can do is to be aware of this if an operation like fill doesn't work.
With the freehand tool you could turn on "close shape automatically" when you need that.
I have a similar problem which is I can create a shape, but it will not fill. It is one of two shapes with a common boundary. I can make those without problem with simple geometrics, but the drawing I'm working on is hanging up.