The Starks are a family of honour and they have given their word that Jon will be exiled. To renege on that would make them untrustworthy and could have ramifications on future deals. It seems likely greyworm would never return, but it's not gaurenteed he wouldn't return if he was told the Starks has betrayed their word. Most of the plot points in the show make sense to me, but I think they could be reordered or more plainly pointed out to make it clearer. My final take was - excellent but not perfect finish.