1) Marriage cannot be enforced and you cannot be coerced into a marriage. However, you cannot keep them guessing and have to inform them of your decision to back out.
2) You can do this directly or through a third party explaining the entire chain of incidents. It would be advisable that you or your parents file an NC/ non cognizable complaint detailing the circumstances you are withdrawing from the alliance and the threats from her and the family including threat to commit suicide.
3) Their response must be reactionary to assumption that you are backing out. You need to choose whether you want to reconsider your decision now. To avoid unwanted trouble, you can offer to partly compensate the expenses they may have incurred in preparation. This must be only part of negotiation.