The story has a lot of potential for comedy -- it is a comedy, isn't it? -- because it takes one step further the premise of "My Best Friend's Wedding". In that movie, the Julia Roberts character attempts to break up her best friend's marriage before he says "I do." In this logline, the guy tries to after the marriage is a done deal.
While the woman is cast as the MC in this logline, frankly, the guy trying to break up her marriage and win her back seems more interesting. He -- not her -- seems to be driving the plot, the one who has to do the hard work of story, will have the better scenes. Consider again, "My Best Friend's Wedding": the female best friend trying to stop the wedding is the protagonist, the one who drives the story, not the best male friend.
So I suggest either make the guy the MC -- or switch roles. That is, make HER the one who invades HIS honeymoon, determined to win him back even though he has already said "I do" to her competition.
Something like: A head-strong woman invades her ex-boyfriend's honeymoon to win him back.
As far as the working title, "The Newlyweds" seems rather prosaic. It's tempting to suggest "My Best Friend's Honeymoon", but that may be too obvious a call back to the other movie. (Although it is safe to assume that a script reader, producer, or director, will associate the two.) Maybe something like: "I Do -- I Don't" -- the diametrically opposed positions of the couple in conflict.
fwiw.