Anything is changeable in the Keep. Just use the spoiler button to reveal. This is helpful when the game doesn't bring over your choices correctly so tiles are still blanked out even though you did do the action. This happened to me with Decent when it said I hadn't completed the main quest, when in fact I had.
As for the romance, I'm not sure if you are talking about a new saved game or going back in your first saved game and playing through from a certain point with a different romance. If the former, then when you finish that one it will copy it over to the Keep but you can then load your old saved game to play the DLC and it will copy that over correctly as well. If you are simply changing the romance within a saved game, then it will over write the old romance with the new one when it uploads to the Keep.
If you go in and change the romance in the Keep then it will make no difference to the game you play in the DLC since it will take the save from your game files not the Keep. The Keep choices for DAI are only relevant to the next game in the series.
I hope the above makes sense.