Guys, I thought I was completely f***ed, because the main quest and this side quest converged and so I couldn't progress. However, after going after some unexplored "?" in Skelige, I did actually reach the Isle of Mists and the game went forward. I kept a save for when they fix the bug to finish King's gambit but other than that, decided to screw it and just keep playing. So yea, you can go to the isle and continue the main story if you want.
Previously, I thought this is not possible, because while visiting some other island, I saw the quest tracker for main quest update, but couldn't really find anything on that island that would be related to the quest, so thought it was some bug. However, when you reach the correct island, the game will actually give you a popup asking if you want to proceed, because you're entering a point of no return.
All in all, since this is possible, I don't consider this bug to be game breaking and therefore start to understand why they are following their production schedule instead of jumping to 1.0.4.1 or whatever, just to fix this.
Do what I did and you can still have your WWW (weekend with witcher)