This is my view and may differ from rest
How many downvotes are required to delete a question?
For questions, a post that no longer adds anything to the site should be deleted.
So, it doesn't actually depend on number of down-votes but most often it depends on the content.
1. Copied puzzles get upvoted?
Yes they do. But only by the people who didn't knew if it was copied. If majority of people think, its copied, it's either closed or deleted as plagiarism.
2. My riddle was down voted because few people motivated others to downvote the question by putting diverting comments which were not required?
I didn't down voted at first, due to the fact that I was not sure if it was within the rules or not. But, majority of downvotes increased when you started editing the question with words like "AD" and it grew when you added another bounty to it asking for downvotes which I felt was sad.
Move on. You have been a good contributor to the site. Mistakes happen. Don't stick to this one and keep making better puzzles in future.
All the best!