Your situation is no different than it is for thousands of other who actually follow the requirements and enter the country legally. And honestly, I only have the word of someone I don't know on the reasons to enter in this manner, and only one view point. Despite all that, it's important not to expose yourself or anyone else to any harm during the process (if you get that far)
It is standard procedure to put detainee's in handcuffs until they are isolated. It's a precaution. Expect it to happen and don't fight it. Don't be scared, it's just a formality. They will remove them once you get to an interview room or detention cell. Remember, they have no idea who you are or if you are a threat to the country and it's citizens (despite you giving your word), so they treat everyone as a risk. You will likely be held in an immigration detention centre for at least a few day or until they establish who you are. If it takes longer to do that, you will likely be transferred to a prison (out of general population) until such time as they feel it is safe to release you or remove you from the country. There is no time limit that they have to release you by. Again, it's important to detail your medical condition in advance so that they can make the necessary accommodations for you.
And Ignore Jee786.....He is being condescending....so no need to respond to his posts.