Your snapped stem will be fine if you let nature takes it course.

I know of some people who insist on growing sativa indoors as its so much easier to trim and harvest and so forth than indica.

They consistently snap the tallest stems that grow up to the lights. It will retard further height growth for the most part but will still fatten and deliver nice flowers.

It is not neccessary to tie her from the ceiling to support the broken branch. As long as one half of the branch is intact, the flower will bloom.

SuperThrive is almost always a good idea.