I have never grown St. John's Wort and so can't give pruning advice, but as far as snow protection, I have made teepees out of 7-8 foot bamboo stakes and wrapped them securely with burlap for protection. Also making a 'sandwich board' with 2 pieces of wood and hinges will protect your plant from the weight of snow.
The article below suggests the plant is short lived and that softwood cuttings should be taken in spring to start new plants. Also, it is cautioned to beware of dermatitis when touching this plant.

Here is a link that might be useful: St John's Wort

One person's treasure is another's bane--St. John's wort is an invasive weed in California. I mow mine, whack it with a mattock, dig it up with a shovel--it still comes back. This is one tough plant and, at least here, will sprout from the roots within a week. I don't know what will happen in your climate but it should be easy to dig up some roots, plant them somewhere and see if they resprout before you cut it back.

The shrub in question is not Hypericum perforatum, a well known weed in the west. It's likely an ornamental hybrid shrub derived from different species entirely. The variety 'Hidcote' is perhaps the most commonly sold. It can be pruned hard in spring if need be.

Here is a link that might be useful: Hypericum Hidcote

I don't believe it's short-lived. I've had one for about 5 years and our winters can be very bad. Mine is now atleast 4'x4'. What about just firmly wrapping it in burlap for the winter to prevent any more branches breaking from the snow?