Answer: General pruning guidelines consider one third of the canopy or branches to be the absolute maximum one would consider removing at one time although this usually applies to deciduous trees rather than needled evergreens. As a rule these junipers are not pruned because they have such a regular and narrow natural shape.
Since the tree top is already damaged, you can try removing it to make a clean cut rather than a break or series of breakages and see how it recovers. There is always a chance however that furture heavy snows will casue the same type of probelm again, especially since the natural pointed shape of the top will not be there to shed snow.
I am not sure why you want to remove lower branches, the tree is so narrow already, but in any case I would not recommend removing the bottom branches if they are healthy and certainly not in addition to removing the top portion of the tree.
Finally, to address work of that magnitude and height, I would suggest you hire a professionally trained and certified arborist to do the work for you. Please make sure they are well versed in finer pruning of ornamental trees. They may also have suggestions as to how to proceed.
In the end, sadly, it may be that the tree is simply no longer attractive and no longer suitable and should be removed. That would be a matter of personal taste and entirely up to you. I'm so sorry about your tree.
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