Nah, I don't think the creators went Bella/Edward Twilight. ; Oh the Gods. Felt canonically underlined that it was being used specifically as a healing act; given its "in the face" narration to make sure the kids had no empty spaces to misinterpret or feel confused about, immortality probably even would've been mentioned during, "and she ruled the kingdom with the same wisdom as her parents." (or etc) Rapunzel very much ages like a normal person (infancy - young adulthood). Anything filling gaps not already filled in that movie is just that.
In regards to Frozen, Disney has done many "Finding Waldo" character inserts in other unrelated Disney movies for years. Notice how Rapunzel/Eugene are nowhere in any of the crowds afterwards, most notably the chapel when the entire audience is laid before the screen, and also where they would've been front-row due to the connections fans theorize between she and Elsa/Anna.
Call me crazy, but I can strangely perceive this as the reason why this happens to be Disney's adaptation of The Snow Queen under the name Frozen as the same reason their Rapunzel adaptation is known to this day as Tangled. I guess giving it a unisex title helped give it a boost, as did see a good chunk of prepubescent boys in the theater when I went to see it. It started with Disney believing that Alvin & the Chipmunks: The Squeakquel ended up outgrossing Princess and the Frog at the box office due to the possibility that demographic I mentioned earlier being turned off from Princess and the Frog because of simply having the word "princess" slapped in the title, and that demographic is somehow expected to associate princesses with girls, along with girls having cooties, a common form of age-appropriate misogyny. Of course, there were some complaints over Disney renaming their Rapunzel adaptation Tangled as it happened, with an example happening to be Justin Chang, a writer for Variety magazine, claiming that it would be like renaming The Little Mermaid "Beached." Although the problem would somehow come from the Disney Princess franchise, I highly doubt that the other Disney Princess films will be given that same treatment, as they were initially released long before that particular franchise came about.
I heard that Rapunzel's, Anna's and Elsa's mothers are sisters. When they died, Anna's and Elsa's parents were going to Rapunzel's and Flynn's wedding but they got caught in a storm. That's why Rapunzel and Flynn are coming to Elsa's crowning. What do you think?
I don't know if this is mentioned in the comments, but there is a headcanon floating around that the queen from Carona (Rapunzel's mom) is the queen from Arendelle (Elsa and Anna's mom) are sisters or related. This is the supposed reason as to the resemblance of the two queens as well as the same general facial similarities between the three princesses.
There are some really cool theories floating around and have been expanded even further. The writers even mentioned that the Queen and King were going to a wedding on their travels (which took place around the same time as the wedding of Rapunzel and Eugene/Flynn)
Now that you think about it, Tangled was made three years ago. Anna and Elsa's parents died in a shipwreck going somewhere very important THREE YEARS AGO. Maybe they where going to see their long lost niece.