It is difficult, by their very nature, for Romantic Comedies to be excellent films. Those looking for a film 'destined to be a classic' probably will not approve of this one. But if you are looking for entertainment, 'The Decoy Bride' has potential.
As far as the story goes, it is pretty standard; you have the famous couple, looking to avoid the paparazzi stalking them, and the 3rd party who gets between them more or less accidentally. What elevates this film is the 'sweetness' of most everybody involved. The audience is not lead to root for or against one of the characters, as is often the case is such films; the characters all have their good points and bad points, and no one stands out as being eminently detestable.
Not only are the major characters likable and well acted, but the location is very impressive. The fictional island of Hegg is every bit as likable and attractive as the characters in the story. The inhabitants speak with a melodic accent which still is understandable, and have an attitude which speaks well for their day to day life. And their ability to profit from other people's foibles.
It has some good lines. For instance, in one scene the mother says "I don't trust that monk" and the daughter quips "They have been known to monkey around". There is a sense of incompleteness to the whole script though; there are rumors it was edited considerably to fit within budget. Still, everything which needs to be there for the story, and our enjoyment, is there.
This might be a good film for the family. There are no swear words, no gratuitous sex or nudity, or implications of rampant immorality.
In short, like most such films the events are highly unlikely and even impractical (like completely rebuilding a castle, probably without permission, in short order and only for a wedding), but it is well enough done that we don't care.
By the way, don't rush out when the credits at the end start. There is another interesting scene to come.