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:

As long as the image file is installed by an official product, and it lives in the CC3+ data directory, it is ok. If it isn't already a symbol, be careful when making it into one though. If it doesn't already have the four different resolutions (_VH, _HI, _LO, _VL), DO NOT tell CC3+ to create them when importing as a symbol, as that would change the image name, resulting in something only you have. Image files must be used exactly as they were installed by the product.

Sure. Copying entities from one map to another doesn't change anything, so if the original maps were suitable for the atlas, then the destination will work fine as well.

Also, both for this competition and the atlas, it doesn't matter if the maps are separate files or one file with several floors on them. In the atlas, I would just hyperlink the former, but both forms work equally well. And the main prize for the competition is for the best building, doesn't matter if it is multiple fcw's or a single one with multiple floors. Also, since the atlas is just an addition to the contest, the atlas maps doesn't have to take the exact same form as the competition version, the rules are just intended for a clear and fair contest.