How to link SVG files to Inkscape?
To answer your first question:

If you control-click (right-click) on an SVG file, select "Get Info" from the menu.  A window will open, with several sections.  One is "Open with:".  You may need to click the triangle next to it, but you should see a selector where you can choose the program which should open the file when it is double-clicked.



Since you want the same behavior for all .SVG files, press the "Change All..." button, and it should work.