Those three restrictions are pretty restrictive actually. I like your idea. I think it's necessary to establish that you leave the Nexus at the time and in the place when you entered it because otherwise there's either a massive plot hole or it's a rehash of "Tapestry" (which is an excellent episode, but the idea has already been done). I really love that idea. I think Star Trek XI used Leonard Nimoy's character in an interesting way that really worked.