Unable to save CustomKeys.txt for WarCraft using WarKeys

No, there is no way to play it in widescreen with the correct aspect ratio.

The Warcraft 3 Editor uses campaigns to share data between multiple maps. In your case simply create a campaign and then import all data (units, triggers, whatever) from the map into the campaign. Then, if you add any other maps to the same campaign you can export data from the campaign out to your other maps.

The only trick to this is that importing and exporting copies data; changes you make in one place will not be reflected in others. So be careful to update your changes in all maps and the campaign if you want to be consistent! The best practice is to make changes to the campaign and then push the changes to your maps: that way you know that the campaign always contains the most up-to-date data.