My love you are NOT a failure. You are about as far from it as I can imagine atm.
You are here. You are asking for help, you are calling the police, you are getting injunctions, you are doing literally Everything in your power to help, encourage and support her. You are doing this knowing the pain she is feeling, having felt it yourself. We all know how EXCRUCIATING it must be for you.
Your DD literally could not have any finer support than you. You have the WHOLE of MN behind you too! That counts for a lot!
IF any of us can help you in RL, please say?
We will be thinking of you, please keep us updated. Perhaps consider both getting away over Easter? A B&B or something somewhere? Both for a break and for the safety, until this monster is picked up?
Can you scale this up to the local police under a honour/racial crime thingy? If you don't get them immediately jumping up and down all over him, write to your MP, Newspaper, ANYONE to publicise this, and get HIM as much under surveillance as you think He has you.
Also agree with the sealing up of the letter box, alert the police to potential postal threats, and call BT/provider and report him for malicious phone calls.
Go for the big guns straight off. No allowance for benefit of doubt.