[akhtene]

As for Beren&Luthien, I don't remember that in their time it was known that an elf could give up his/her immortality. It was after Beren's death that she sang before Mandos, and then Manwe gave her the choice.
So imho Thingol was simply trying to get rid of an unworthy in-law-to-be (in case he refuses or gets killed). I can't imagine he believed that an unhappy mortal could stand up to a Vala.

As for Elrond, the quest he set for Aragorn was more realizable - with Elrond's advice and his sons' assistance - and besides, it had to be done. Elrond showed his wisdom, securing the future of his only daughter (as well as he could) and perhaps motivating the greatest Man of the time to claim and win what was his by right.