If Thungni knew the answer to Durin's Consternation, I strongly doubt he'd have let his son destroy himself over it. Although the Ancestors left some puzzles behind, I don't believe Thungni would fail to teach something so fundamental to runecraft if he knew it. And I suspect that for things like gromril chain the reason Grungni didn't teach it is because his method couldn't be replicated by mortal dwarves. I think Durin's Consternation comes from a lack of theoretical knowledge that he could have passed on if he'd known it.
Or he might have taken the approach that some knowledge has to be learned and discovered by the individual Dwarf, and so was letting Durin conduct his own research. Or was waiting to teach Durin more, after Durin expanded his own repertoire, but Durin wasn't able to get far enough on his own. Durin's Consternation may just refer to
Durin's confusion and inability to understand a thing.
Or, for another approach and simpler answer; he simply didn't
know his son was going to get himself killed over it!
We don't know if that is what Durin destroyed himself seeking, and the Ancestors are not perfect. Ancestor Gods they may be, they are not human. They are Dwarfs.
The Ancestors did not fundamentally change Dwarf psychology. Dwarves were always secretive. Dwarves were always distrusting. Dwarves always had a thing about grudges. Dwarves always held a tendency for tradition. The Ancestors just focused the Oaths around more productive means and taught the Dwarves various things which helped them a lot. This was, of course, a big deal, but there is a difference between fundamentally rebuilding Dwarf society and completely ignoring Dwarf psychology.
There is nothing suggesting that the colonies closed themselves off, we only have evidence saying Ulthuan basically forgot about them during the Sundering and they decided to united as one Kingdom, which Caledor II seemed to have smacked down.
Yeah, this.
Alratan seems to be taking an approach of "Well, whatever the Ancestor Gods do is Dwarfly, and even the things that weren't part of Dwarf society before they did it, became part of Dwarf society after the Ancestor Gods did it; therefore, that means you are free to lay down precedent and it'll become Dwarfy."
Hobbit and I take a more cautious and skeptical or conservative approach to things though. And also believe that it's not
just a matter of "Well, they did it, and so it
became Dwarfy" but consider the possibility that they were working
with Dwarf psychology, traditions, mores, ideology, ideas,
things in general, when they did what they did.
That it's not just as simple as: "Well, whatever I trailblaze becomes the new established and traditional path. So therefore, I am free to trailblaze as I want. All that matters is
success. If I succeed, or if I do a thing and it becomes popular enough, then that counts as success and is properly Dwarfy. QED."
Where you start from, how you do a thing, how you get there, and
where you get... all that matters. Both the journey and the destination matters. The means and the ends all matter.
In fact, even
more care should be taken for anything precedent-setting or trailblazing-y. If something you do might end up having societal effects on the level of an Ancestor God? You had damn well better be damn sure about making sure that you leave good effects and echoes. You can't just do a thing and assume it'll be okay because it happened and you succeeded and it altered society or set new precedent so therefore its fine right.
How secretive a dwarf is is still culturally constructed. We had the opportunity to start shifting dwarven culture in the Far North a small way towards being less selfish and secretive and more altruistic and sharing. We chose not to take it, but the option was still there.
Or maybe we chose to keep Dwarfs more individualistic and free, focused more on family over centralization of power/authority. Individual (or family-based) liberties. Those are good things too. Those are virtues and values too, y'know.
It wasn't just a choice between "obvious good thing and obvious bad thing", where you know what the obvious Greater Good is and we
failed to pick the Obvious For The Greater Good Choice and failed to progress towards a more enlightened society and thus failed the cause of positive change and enlightenment.