Don't know if anybody has suggested this, and I can't be bothered to read 22 pages of comments, but I believe Victarion will win the battle for Barristan, finds Dany gone, and will kill Barristan and steal a dragon and goes back home. Tyrion may be able to tame the remaining dragon by the time Dany gets back and proves his worth to her, then they, her Unsullied army and new Dothraki army finally heads to Westeros.