[Voralphion]

I don't think that Elrond actually could have thrown Isildur into the fire even if he had wanted to. Isildur was one of the mightiest of men at that time, being born and raised in Numenor. He was very strong, and coupled with the fight he would have put up to protect the ring, as he was under its power, Elrond wouldn't have had much of a chance to throw Isidur in.
Elrond couldn't have known what would have resulted if he had let Isildur leave with the ring. He probably thought that Isildur would eventually come to his senses and understand that the ring must be destroyed, which he did. He can't have known that Isildur would be attacked by orcs on his way back to the north and that the ring would be lost.