[RosieCotton]

BUT, was Elrond already a ring-bearer at that point in the story? - or did his ring come later? To me, that may change everything about whether he could have taken out Isildur at that moment.

Do you mean that because HIS ring was subject to the One, he was somehow under Isildur's power? That's an interesting thought. Or do you mean something else?

[HairOfTheDog]

Well, It could affect his ability to resist it yes!

On the other hand, it might also have given him sufficient personal power to take out a "fledgeling" ringbearer.

I don't know enough about the ways of these rings...