Here is a fixed version of the file. I've set it up with a 50' grid with 5 snaps.

Here's the exact steps to set up a reasonable grid:

1. Right click the Snap button in the lower left.
2. Hit the New button in the dialog to define a new grid
3. Select 2d Rectangular as the grid type and hit continue
4. You can name the new setting whatever you want, but it is best that it follows the actual settings, so I named mine "50, 5 snap". I then set the snap divisions to 5, and the X and Y spacing both to 50. What this ends up doing is making a snap grid that has a visible dot every 50 map units (feet), but it snaps every 10 feet. If you feel the the snaps are too fine grained, try setting the snap divisions to 2 instead, and if you have too few snap points, try with 10 for snap divisions.
5. Click ok, and then back in the select grid system dialog, scroll down and click on your newly created grid to select it, and then hit ok. This will make your new grid the active one. Save the map.

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.