So it looks like you've got some stuffed up meshes and missing textures, as well as the gates for Watermill being unable to load.
First of all, I'd find and remove the downloaded objects from your files and see if FH will start up without crashing. Don't worry, this shouldn't mess up your map, the objects will simply be missing. If FH starts again, then slowly add back the objects individually and keep restarting FH until it crashes again. If it crashes after an object is re-added, then you've found your problem mesh, and if that's the case then you'll just have to exclude it from your map.
When you downloaded the meshes you used, did you make sure they were saved AND exported from Object Maker? An object not exported from Object Maker will explain why it's missing from the map in-game but not when in Map Maker.
As for the missing materials, try going through your files and seeing if you can locate them anywhere, they may have been put in the wrong folder or aren't where they're supposed to be, and need to be moved.
And you can save your map's files by going to media>terrains and finding a folder with your map's name on it. This has all the .cfg files for your map (ex objects, gates, terrain textures, etc). There should also be a .map file with your map's name on it (this is the actual map). You can save the folder and .map file elsewhere so that it can all stay intact if you need to uninstall FH, then just add it back in and everything should be where it should. EDIT: I forgot, don't forget to save your heightmap and mask image files as well! Be sure you back those up with the folder and .map file.
The Watermill gates I'm unsure of, I'm going to assume this is because of FH crashing, or the Watermill map itself has become corrupt with this turn of events. Any corrupt file should be fixable with a clean install (most of the time, but sometimes not).
Last thing, make sure you DON'T click your mouse while a map is being exported. This could either cause FH to freeze followed by crashing, corrupt the map files, or prevent FH from opening again. While a map is being exported, make sure to not click anything and just let the process run through.
