Easiest is to just hide the sheet you don't need, this ensures things doesn't get in the way. You can do more complex selections too, look at this article for more about that:

The break in the wall is a permanent edit (In most cases it will divide the wall into two separate parts.)

If you change your mind right after breaking it, you can always use undo (ctrl+Z), but if you have done other edits in the meantime and undo isn't appropriate, you need to manually edit the wall. In most cases, just using the node edit tool to bring the end node of one end over to the other works good enough, although it might end up leaving a line at the break point due to the fills not lining up seamlessly.

Other options include just drawing a small wall-piece into the hole, or just simply delete the existing wall and just redraw it. That's usually quick using a wall drawing tool.

You need to draw the forest around the lake. The sheets are arranged so that forests will cover up lakes and rivers. Moving things around to make the lake appear on top of the trees will look very bad, because the water will be clipping the treetops in a very unnatural way.

You can either do this manually, or use the exclusion commands to have the trees placed around the lake:

Don't use the line tool if you want filled entities. Remember, CC3+ is a CAD-based drawing tool and is all about the entities, it isn't like a painting program where you can just flood fill any area that looks enclosed, you need to have an actual entity wherever you want fill.

Use something like the :CC2POLY: tool instead. (And make sure line width is set to 0 when you draw it, otherwise only the outline will be filled.)

You need to create a floor drawing tool using it: