Just click on the ring in the viewer and drag it smaller (it will allow you to create a "negative" feather by dragging inside the control point ring). You can click and drag on the "ring" portion of the control shape also. By using the red line part of the shape to resize, it will constrain the proportions of the entire mask as you resize. There is an "extra" dot control on the top of the shape and if you drag it out to the left, the circle shape changes to a rounded rectangle and finally to a rectangle when dragged all the way to the left.
The shape masks can be animated by keyframes as well. Once you set an initial keyframe, the size, shape and rotation of the mask will animate to follow your changes over time.
I was trying to recommend that you only use the blurring on "light" areas like sky and decrease the amount of blurring (or set it to zero) over darker areas (or scene objects. The spots are not as bad over darker areas. For the darker areas like those trees/shrubs, try using a Colorize filter (in the same way as you're using the Gaussian Blur) and set the Remap Black to a nearby shade. Remap white to white (it's default is a beige) or another shade of a neighboring area. Colors can be keyframe animated in FCPX so that as the scene changes/lighting changes, you can use the same effect and change the remapped colors as time goes on.
What you're attempting to do here is ultimately more time consuming than reshooting all the video. There are film cleaners that you can use to remove a lot of the hairs and dust (care must be taken not to let the film touch sections that are still wet - so - another time consuming task.)
Are you shooting directly into the projector lens? or refracting the image to a screen (it looks absolutely flat, so I don't get the impression you're shooting off a wall or straight projection off a screen.) I'm asking because I had a "Moviestuff" telecine projector, but an accident with a mis-feed took out the film gate. I'm looking for another more economical method of transferring my old 8mm films and you're getting very good results!