Also, this is really important: Be careful when you edit your UVs by hand when the sync is on. Its better idea to not do it at all and just edit when the sync is off.
When you select an edge or vert thats at the border of an island – which is the seam – its also going to select the same edges or verts at the border of the neighbouring island (neighbouring in the viewport, in uv layout the islands can be placed wherever). Since the seam is between two islands, one vert on the seam in viewport corresponds to two or more verts in the UV layout. So if you click on just one seam vert in the UV layout, its going to sync with the viewport, and then sync back and select all the corresponding verts.
I’m assuming thats the reason why island select or other tools are disabled.
