1. Someone on the other side perhaps Nephi in a ‘postmortem’ state provided the anachronistic Isaiah material.” (I'm not sure I follow this line of thought. It it assuming that this person provided a more accurate version of Isiah? I'm not sure how getting help from an Angel would explain problems with Isaiah in 2nd Nephi any differently than getting the translation from the Urim and Thummim.
2. Jesus could have intentionally communicated the Isaiah anachronisms to Mormon and Moroni.