Tolkien said they were the same Elf in later writings even though LotR and Silmarillion contradict this. Apparently Glorfindel came back during the second age by the Valar to be a proto-Istari type thing.
As a matter of interest, can you remember where Tolkien said that? I don't disbelieve you, I'd just like to know how he justified it. I don't think LotR or the Silmarillion contradict the "one Glorfindel" hypothesis, but they don't support it either; as far as I can recall, neither book says anything about Glorfindel outside his/their immediate context in the book.
Why was Elrond not sufficiently powerful to defeat Sauron? Elves of earlier ages performed feats that would make that a walk in the proverbial ash torn tundra.
Generally, elves are less powerful than Maiar, even in the First Age - Finrod Felagund could not defeat Sauron, for example. Also, there's a sense that the Elves' powers are weakening with time; I don't know whether this applies to individual elves, but collectively they are less numerous and less powerful as the Ages go by.
Why isn't there a LOTR: Return of FIGWIT movie, a feature length film encompassing the trials and tribulations of this elfs six or seven seconds of screentime?
Don't say it too loudly - Peter Jackson might hear you