[Aiwendil]

RB-DF-07.5: Hmm, I think some of my previous post got lost somehow - I meant to propose this emendation:

Neither Orc nor Balrog dared ever after to pass over the mount of Fingolfin or draw nigh his tomb, until the doom of Gondolin was come and treachery was born among his kin. Morgoth RB-DF-06 {goes ever halt of one foot since that day, and the pain of his wounds cannot}<LQ2 went ever halt of one foot after that day, and the pain of his wounds could not> be healed; and in his face is the scar that Thorondor made. <GA There was lamentation in Gondolin when Thorondor brought the tidings, for many of the people of the hidden city were Noldor of Fingolfin's house. Now Rochallor had stayed beside the king until the end, but the wolves of Angband assailed him, and he escaped from them because of his great swiftness, and ran at last to Hithlum, and broke his heart and died. Then in great sorrow Fingon took the lordship of the house of Fingolfin and the kingdom of the Noldor>, and he maintained still his realm behind the Shadowy Mountains in the North.

By taking more of the passage from GA, this avoids editorial additions and redundancy while not, in my opinion, omitting any important details. Okay, so it doesn't literally include the detail that there was lamentation in Hithlum, but it does single out Fingon's particular sorrow, which actually is a detail we lose otherwise.

Or we could take the same approach but move the sentence about Gondolin to the point Findegil suggests. I think that rather than changing 'Gondolin' to 'Tumladen', however, we could just remove the second 'Gondolin':

And he laid him upon a mountain-top that looked from the North upon the hidden valley of Gondolin. <GA There was lamentation {in Gondolin}there when Thorondor brought the tidings, for many of the people of the hidden city were Noldor of Fingolfin's house>; and Turgon coming built a high cairn over him.
Neither Orc nor Balrog dared ever after to pass over the mount of Fingolfin or draw nigh his tomb, until the doom of Gondolin was come and treachery was born among his kin. Morgoth RB-DF-06 {goes ever halt of one foot since that day, and the pain of his wounds cannot}<LQ2 went ever halt of one foot after that day, and the pain of his wounds could not> be healed; and in his face is the scar that Thorondor made.

<GA Now Rochallor had stayed beside the king until the end, but the wolves of Angband assailed him, and he escaped from them because of his great swiftness, and ran at last to Hithlum, and broke his heart and died. Then in great sorrow Fingon took the lordship of the house of Fingolfin and the kingdom of the Noldor>, and he maintained still his realm behind the Shadowy Mountains in the North.

About the Shibboleth: I am still inclined to think that the changes to the sequence of events around Miriel's death and Finwe's remarriage fall under 2b, proposed changes that would require too much tampering for us to implement. But that discussion should really wait until we revisit DoV. The other relevant principle for some points in the late writings is 5:

Information in sources of lower level priority is to be preferred over information in sources of higher level priority where the item of information in the source of higher level priority can be reasonably demonstrated to be an error, whether a "slip of the pen" or from inadequate checking of previous writing.

It still seems to me quite likely that some of the apparent deviations in the late writings are simply due to forgetfulness, rather than a deliberate changing of the story. Principles 2 and 5 don't dictate exactly where we draw the line between what to take up and what to reject, but they do frame the debate that must occur on each specific point.

is this really an english proverb?

I think there was a typo in Ekimeniso's post; the proverb is 'rule of thumb'. I think the German equivalent is 'Faustregel'.