What happens here is that CC3+'s drawing tools are set to stop at the map border, and the map border itself is identified by entities on the MAP BORDER layer. What has happened here is that you have somehow gotten some extra entities on that layer, which basically redefines the map border.

Open up the layers dialog by clicking on the L: indicator in the status bar. Hide all layers except MAP BORDER. It should now be quite obvious which entities do not belong here. The map border itself should normally just be a thin outline around your map, there shouldn't be anything in the middle. Now either :CC2ERASE: the offending entities, or just use :CC2MCHANGE: to move them to another layer.

A very nice map. Looks great. I've always been very partial to SS2 myself.

Not as a finished feature built into the program, but you can do this using a Macro.

The Large Exports annual contain such a macro with explanations (the end result with this macro is to combine the individual tiles into one large image, but the start of the macro exports the map as smaller tiles.)

Check with support. If you already own CC3+, they might provide you a copy of CC3 or find another way to provide you with a solution for getting the symbols.

That CAD Blocks site has a lot of good stuff. I've added it to the list in the Free Symbols resource thread.

These CAD blocks are actually ready to use symbol catalogs, which is very neat. I first thought I would have to resort to just copy/paste from the drawing, but load them up as a symbol catalog, and they show up as proper symbols.