Two ways (bulleted instead of numbered here because I can't seem to get the numbered list to work right now):

• In the Sheets and Effects dialog there is a checkbox above the list of sheets called Auto hide all sheets except current and COMMON. Check that box and hit the Apply button bottom right. When when you click the next sheet down the first will hide itself, and so by working your way down the list you can see if a sheet has something on it. Be careful not to delete a sheet called SCREEN if your map has one. That sheet contains a white polygon around the outside of the map that masks anything that continues outside the edge of your map, like a symbol or the tail end of a road or river - or probably in your case, hex halves.

• With only the BACKGROUND sheet visible (but no check in the Auto hide... box) right click the Display Speed Settings button (the hourglass on the left) and you will find a block of commands at the bottom of the list that will help you do the same thing. It's slightly less efficient since you must right click the button to get the menu each time you want to look at the next or previous sheet. The advantage, though, is that you don't have a huge dialog blocking your view of the map.

Do you mean something like this?

I notice that the land in that image is also very dark. I can see a potential problem arising if you want to remake this map with everything darker than this relatively pale style, since it's very difficult to make the symbols darker. If that's where you are going with this map it might be better to chose a darker style to start with. However, no other overland style has such an extensive range of symbols as the Mike Schley Overland style.

I hid the COASTLINE sheet and added effects to the BACKGROUND sheet and the LAND sheet. I haven't deleted any existing effects.

Here is the FCW if you want to look at what I've done.

That's great. It was a suggestion option, but I chose SIMPLIFY because it's guaranteed to get rid of most of all those problems all in one go.

Well done for succeeding with the more tricky solution.

You only need the one for CC3Plus. Other executables are for older versions of CC that are no longer sold, but which some of us old timers still run every now and then.

It will probably remain a mystery forever now, but at least I know what to suggest trying with anyone else that has a similar problem.