I think peoples fear is that a very small minority of people effectively will have control if theres an SNP coalition. I understand where you are coming from on Scotland being subjected to a Tory government but the argument is flawed in that the majority of Britains voted Tory, the majority will not vote SNP and theres a real fear they will just be out for themselves/Scotland not the entire country.
Lib Dems have had their critics but they haven't held back the country to further their own objectives in the last 5 years.
Now if the people of Scotland do not wish to be controlled potentially by people elected south of the border they have had several chances to correct this, we south of the border have never had a choice. 5 million Scots cant control the power of 60 million others.