[Mithalwen]

I had also assumed it was Legolas' keener sight that led him to first identify the Balrog and I think that this is just a case of the name being recycled.

Tolkien did recycle names and unlike with the case of Glorfindel where the coincidences between Glorfindel of Gondolin and Glorfindel of Rivendell were too strong to be ignored there is not the same evidence.

Legolas of Gondolin if he were avaliable to be part of the fellowship would have been of a stature to compare with Glorfindel and it is clear that Legolas of Mirkwood is not "an elf Lord such as Glorfindel". His behaviour does not fit with being a veteran of Gondolin. Also Legolas of Gondolin would surely have fought at the last alliance yet Legolas of Gondor has not travelled so far. Much more consistent with being either unborn or left behind as heir while his father and grandfather fought in Mordor.

Legolas can be old enough to regard Aragorn and Gimli as children without being a first age Elf.