Hmm, good questions.
The Silmarillion of the later 1930s notes that (section 28, end of the Quenta Silmarillion, The Lost Road) among those of the Eldalie that did not forsake the Hither Lands
'... were Maglor, as hath been told; and with him for a while was Elrond Halfelven.'
This was changed in the 1977 Silmarillion, as 'with him' refers to Gil-galad there rather than Maglor.
As for dating, making a composite of sorts (!) based on
The Tale of Years found in
The War of the Jewels, it looks like Tolkien considered that the sons of Elrond were born in year 532 of the First Age and were taken captive in 538, with the War or Wrath taking about 42 years (545-587), and the end of the First Age dating at 590.
If that helps any