One idea about parallel sort is that perhaps if multiple workers feed data into the sort, they can each just sort what they have and then merge the results.
Sounds like a good approach for parallel sorting, however small extension
to it that could avoid merging the final results is that workers allocated
for sort will perform range-based sorting. A simple example to sort integers
from 1-100 will be, worker-1 will be responsible for sorting any integer
between 1-50 and worker-2 will be responsible for sorting integers from
51-100 and then master backend just needs to ensure that it first returns
the tuples from worker-1 and then from worker-2. I think it has some
similarity to your idea-5 (use of repartition), but not exactly same.