jedibuttercup

>Tolkien had absolutely no problem using names over again.

I don't dispute this point. Legolas, for example, is also the name of a Gondolin elf, yet no one ever confuses him with Thranduil's son. But their descriptions were clearly different; Glorfindel, not so much.

>After that, there is no record of any elves (other than the coming of the Istari) returning to the shores of Middle-earth.

Maybe not in the Silmarillion itself. But Christopher Tolkien himself has said that isn't an authoritative work; he left much out or invented whole passages in order to bring something to publication, and regretted it later. Hence the publication of the "History of Middle Earth" series, containing ALL of JRRT's compiled notations, including his original intentions, unfinished or not, for most of the tales.

>Thingol had the chance to go to Aman and returned to Middle-earth once; one would think he would do it again if able.

Thingol wouldn't return without Melian, I don't think. And why would any of the others wish to go back when they'd lived hard lives and died horribly, and much of their families were still in Mandos or had sailed over? Remember, Finrod's wife did not go with him into exile to begin with; Amarie would be even less likely to follow him back for a second trip.

Even Glorfindel may not have wished to go; I don't find it unlikely that he could have been charged with continuing to protect the line of Turgon (as he did in the escape from Gondolin: the princess Idril, her husband Tuor, and their son Earendil), since that King's great-grandson (Elrond) would be such a huge influence all throughout the Second and Third Ages, a critical force in the overthrow of the Shadow.

Even leaving your points aside, though, just from the trilogy it's clear Glorfindel isn't just any elf. In the chapter "Many Meetings" Gandalf is explaining to Frodo how Glorfindel was able to glow so brightly: "...those who have dwelt in the Blessed Realm live at once in both worlds, and against both the Seen and the Unseen they have great power." In other words, Glorfindel has been to Valinor, which means he was either with the very old elves who left Aman for ME in the Noldor rebellion, or else has come back by way of Mandos, or both. But even if you assume only #1, that would mean Glorfindel of Rivendell had lived at the same time as the Gondolin one, and Tolkien never used the same name for contemporary elves that I can tell.

More convincing, however, might be an essay published in the "Peoples of Middle Earth" volume from the HoME series, originally written by JRRT in 1972. In it, Tolkien declared that the two Glorfindels were in fact the same, and that Glorfindel would have had to come back in the Second Age since no elves could return by the sea route after Numenor was destroyed and the seas bent.

In the same essay is also explained more about Glorfindel's power; it's suggested that "as a reconstituted Elf he would have become closer to a Maia in power than any normal living Elf." Not just any elf could have scared off the Nazgul, or (as mentioned in the appendices) face off against the Witch-King of Angmar in full battle and give the prophecy about that being's downfall.

The reason a lot of people still believe they're two separate individuals is because Tolkien did not write that essay until after the trilogy was published, and because Christopher didn't make it public until so recently. So people decide to disregard it as "not authoritative".

Personally, I don't think it matters which you believe; I just find the evidence heavily on the "one elf" side, and I also think it makes his story that much more interesting to write about.