Arwen's Mortality

[Macalaure]

His children - with a renewed Elvish strain, since their mother was Celebrían dtr. of Galadriel - have to make their choices.

I've been thinking about this for quite some time now. This is how I understand it by now, I hope somebody can tell me whether I'm on the right track.

First I thought that Elrond's and Elros' situations are symmetric.
Elrond chose to be an elf. Then he marries an elf. My logic told me that normally elf+elf=elf.
Elros chose to be mortal. Then he marries a mortal woman. Why didn't their children get to choose with a renewed mortal strain? (which was also Farael's question, I think)

I think it is stated in 'Laws and Customs', that if mortal and immortal blood is mixed, the outcome is always mortal. So Earendil and Dior and his children were mortal half-elves in the beginning. Then the Valar had the brilliant idea to let Earendil and Elwing and their children choose between the fates of elves and men. Does this now mean exactly that, that Elrond and Elros chose their different fates, but they still remain half-elves nonetheless and don't actually become Elf or Man? If this is the case, then it is clear: Elrond's and Elros' children would always be mortal - unless new immortal blood comes into play. This renewed elvish strain via Celebrían then seems sufficient reason to give Elrond's children a choice, too.

Do I understand it right?