User blog comment:Omega natsu2/Chapter 385: Spirit King vs. Demon King, Predictions./@comment-7418318-20140527171131/@comment-7418318-20140527182214

Well... Solomon does have the ability to see and manipulate this second layer, but doing so where he is and erasing the borders between those dimensions are quite different feats.

Regarding the Djinn and Goi matter: I'm not entirely sure if the Djinn only accept Goi. I understood Yamuraiha's words as meaning that mages can't (or are at least very unlikely to) become household members. But even if trained mages can't become King Vessels, I wonder if it's due to their bodies or due to the way of thinking they aquired during the training. Or if it could be the same reason why the Djinns refused to allow Sinbad to conquer any more dungeons: They might think someone who's a mage is already powerful enough and thus shouldn't get the additional power of a Metal Vessel or Household Vessel.

Considering everything we know about Sinbad (particularly his abilities in magoi manipulation and what Yunan said about him at his birth) I wouldn't be surprised if Sinbad would have been able to become a mage, if he had received mage training instead of capturing a dungeon first, so I think the bodily thing least likely of the three theories.