[Mithalwen]

I agree with Galin. The valar felt the Gift of men from Eru could not be withheld to those entitled. It isn't simply a matter of death since as has been pointed out that Elves are only immortal in the sense that their life is bound with the existence of Arda. It is the possibility of passing beyond the circles of the world and having more scope to decide one's own fate as well.

It seems to me that mortality was the "default setting" for half elven by blood. Mithrellas's children did not get the choice and she surely abandoned her family to avoid the pain of seeing her children age and die. Earendil and Elwing clearly developed as humans not elves since they married in their early twenties and had children at an age decades before Elves would be physically mature and of age to marry. Elronds children are of course more than half Elven and their development may have been more on an elvish timescale as the children of two immortals (the spacing between their births suggest this). The grace of choice extended to them must have been a very mixed blessing and may well explain why Elrond delayed marriage long after meeting and falling in love with Celebrian. Embarking on parenthood knowing that you may be separated eternally from your children must have been appalling and makes her suffering all the more poignant I think.