Actually, as long as it is for non-commercial use, the license allows you to have it installed on up to three computers at once. (Computers belonging to your household, you can't go and share a license with two friends)

I recommend starting with the Symbols and Symbol Management chapter in the User Manual (accessible from the help menu), it explains the details of how to create your own symbol catalogs, and then how to import .png's (recommend reading the entire chapter, not just the importing png part inside it, since that part relies on what has been explained earlier.)

The Tome do go into greater detail about your options, but the User Manual should get you started.

For the second point, CC3+ scales to paper on print. The map itself should be scaled according to the actual real world size, not the print size, so if you wish to cover an are 500 by 500 miles, set the map dimensions to 500 by 500.

In the print dialog, you can then specify desired print scale when you print, or if you export to an image, set the desired pixel size as needed by your print.

Not as long as it is attached to the drawing. That embeds the palette into the map itself instead of relying on the external palette file.

But you can also keep it around under a new name, and then manually load it using PALLOAD if you ever need to.

When you save a custom pal, it saves a file called fcw32.pal in your CC3+ data directory. If this file is present, ALL maps that doesn't have an attached palette will use this instead of the default palette. So the solution is to delete or rename this file, and CC3+ will revert to using the default again.