I can answer your question from the perspective of an innocent person who has been interrogated by the police.
1. The police do not have any legal obligation to read your Miranda rights, they must only do so when they are going to use information they gather in court.
2. Even if the police do not read your Miranda rights, they can claim that they did and then turn it into an issue of a police officer's word against an accused's word, in which the police are always viewed as the authoritative answer.
3. The police are only limited in their ability to interrogate to the degree that they can keep any questionable tactics to an issue of the police officer's word vs the accused's word. (so long as they don't leave any video evidence, witnesses, or physical scars, they can interrogate you however they please.)
4. The police already use "enhanced interrogation" techniques similar to those used on terrorism suspects and do so with impunity. (in my case, I was denied medical attention and then I was stripped and forced to stand naked and bleeding while interrogated by police. No lawyer will touch the case, therefore I can only conclude that such tactics are defacto legal to perform on any suspect).
This may not be the "legal" answer you are looking for, but it is the real world reality of it.
(the news article corroborating my claim of innocence here, my story here)