Yeah, I like all of your ideas. True heartbreak killing them sounds like a great twist, and I like using the bubbles/sea foam thing from the original Little Mermaid. Her great-grandmother was taken by humans, and her great grandfather could have been close to death from heartbreak, but maybe his anger and bitterness about the humans saved him. Meaning in order to save themselves from their broken hearts they have become, well, heartless. 

And yeah, them picking what age they want to stop is good, so leaders can still look strong and dignified and experienced and everything. Then after many, many years of this constant age the aging starts again. 

And yeah they wouldn't be invulnerable to death, just a little more resilient. They can still be killed, but if they take care of them selves they can live for well over a hundred years. 

And I like the idea that the slowed aging affects a bonded human. Maybe merpeople mate for life or something? So human's aging will also slow, and maybe they become more resilient to illness and disease or something? Quicker healing in water (which could also be another mermaid ability that protects them from predators drawn by blood)?

I agree about the feet and legs thing. Let's do it.