Only one lineage?

[Voralphion]

But should Elladan and Elrohir have descendants, they will all have to choose-- until somebody chooses mortality, and then, no more choosing after that.

No, if they chose to be elven they would eventually go into the west and any descendants there would have to be immortal as mortals weren't allowed in the undying lands.
I also have to agree with others in that Elwing, Earendil and their offspring were only given the choice because of their great deeds. Any other halfelven would be mortal as any mortal blood = mortal.
The choice was only given to the offspring of Elrond not the offspring of Elros because although Elrond was an elf he still had some mortal blood which would have made his decendants mortal, as any mortal blood=mortal. This choice didn't kick in however until Elrond left ME and so his children were considered to be immortal until he left, when they had to decide for themselves. This explains why Arwen had such a long life before she chose mortality.