I gave a look to your model, and noticed that you applied your subsurf/multires modifier, giving then your mesh so many faces that the unwrap code will always have big difficulties to make a clean unwrap.
a solution if you don’t have anymore your work from before you made the mistake of applying the modifier :
-Go to Object mode
-Add a Decimate modifier, set it to “Un-Subdivide” and increase the iteration to 6, then apply it
- In Edit mode select all , press CTRL+E -> Clear Seams
- Now select the edges you want to make as seams and CTRL+E -> Mark Seams
From there the unwrap will be easier for the Blender unwrapper and will be clean.
That’s something to remember : always unwrap before applying any subsurf/multires modifier to avoid that problem