The dark edge at bottom and right has been there since day one (the first bug report is from 2000). It's not to say that it's correct behavior, but it's unlikely to go away any time soon.
The gaps in the river is a buglet caused by the river hitting the edge of the map. It's supposed to be clipped at the edge, but it seems that FT is just dropping the last segment. Grids have the same issue, but in that case it can be eliminated by turning off adaptive grid resolution and bumping up the number of segments. These issues are easy enough to correct onscreen in FT, but those tricks don't work on export to CCx.
If you're using the multiple file export feature, have you tried using a bit of overlap between images (say 10% or so)? That way the rivers at the edge won't have a problem with missing their last segment and the black line can be eliminated fairly simply. Not ideal, I understand.
I would rather just export a huge single image, but the problem is that FT would have to compute a strip of the image, draw the raster overlays, draw the vector overlays, and then write out that strip. The clipping issue can cause problems with this operation, unfortunately.
A suggestion was made to have a different file format for the 64-bit and 32-bit versions that would incompatible with each other, but I am of the opinion that there should at least be an upgrade path from 32-bit to 64-bit. Making that work would result in a file format that was compatible between the two versions with the possible exception of data sizes.
The tools should already be feathered. The circular painting tools should start at 0 at their edge and rise to the user-specified value at the center. For the more advanced users who want to try something different, FT offers the brush preset file. If you save a brush preset file and then open that file in Notepad, you may find that it's a simple text field that contains a number of interesting values. If you've ever used the Edit Paintbrush Settings dialog in Wilbur and saves a preset, then this set of features will look familiar (FT and Wilbur share a brush engine). The part that might be of most interest to you is the "FileName" field that lets you specify an image file to use as a brush. I recommend setting up a brush you like in Wilbur and saving the brush preset into the FT folder. Then you should be able to use that brush in Wilbur. I forgot to port over the UI for the brush parts into FT, but the engine is there and it works the same (I think brushes are upside-down in FT realtive to Wilbur, but that's a minor thing).