In the Netflix SF series Travelers, they get around the problem of sending a human consciousness, i.e. someone's mind, into the past with a quantum "stack". What this does has something to do with entanglement, plus some unspecified way of transmitting information across an Einstein Rosen bridge (or something). But objections aside about whether or not a future human race will build such a thing, what else is the problem about the idea that transmitting any kind of information back into the past can alter it? (Especially if that information means you can "inject" active human agents into your past with missions to carry out changes to that past?) How would you determine the success or failure of your agents?