... can you please show from the "Letter of Understanding" that it is an agreement.
A JW parent who signs this letter of understanding is agreeing to terms of service of the institution, one of which is that medical staff may administer blood transfusion if necessary. This is not an agreement to accept blood transfusion. It is an agreement on the terms which the institution will provide services to the patient.
...can you please explain, if you know for a fact , that the same document is being used by the Watchtower?
"Used" has many connotations. The document is known by Watchtower appointees and is asked for by those appointees for sake of JW parents having the opportunity to sign it. But you should know this by now. So what is your question for? Watchtower representatives have also encouraged other medical facilities to develop/adopt similar documents.