The getting in touch every 10 years is to exchange new lore discoveries and discuss any major events that have occurred in the clans in between time. The basic traditions and customs that the Dalish follow were established back in the time of the Dales. That is what they are all meant to be following. It is not a high school reunion, with people living separate lives according to different customs in their own homes, and just having a small commonality because they went to school together. It is a whole way of life. Look at the Jewish people. They have been scattered to the ends of the earth but when two Jews come together, they can recognise one another by the common traditions and culture that they have maintained down the years despite never having got together. There are some variations between different groups but not in core beliefs and traditions. That is what I am getting at with the Dalish. There would have been certain traditions and beliefs that would have been what they took from the Dales and before they split up into smaller clans, it would have been agreed that this was what they were trying to maintain above all else in remaining free. It is their racial and cultural identity. It matters to them. If a clan feels free to just willy nilly change things that is not being Dalish.
This is historically illiterate. 'Jewish culture' is also extremely diverse and frequently internally contradictory, famously so. At many points in history has been highly fragmented as groups thoroughly sought assimilation in widely different societies. 'Jewish culture' as a singular is more the product of anti-semitic conspiracy than fact.
Jews in most of history, including the present, have not been interchangeable with eachother in core beliefs and traditions because their core beliefs are, like most population groups, determined by culture and heavily by where they live and the communities with which they reside. Assimilated Jewish Germans had far more incommon with Germans than they did with Jews of France, who were more like the French than the Jews of the Middle East, and so on. The belief that they were all alike with a common, alien identity is the product of anti-semitic conspiracy theories of international jewish cabals and resistance to the sort of social integration that Jewish communities frequently and actively sought. Jews have a shared religion and traditions, but cultural values are shaped by communities and the Jews were (and are!) as diverse as the many nations they've assimilated into. So are their interpretations of their shared faith- the phrase 'two jews, three opinions' exists because there isn't a singular interpretation of religion or identity that all practicing Jews fall into. There is constant, vibrant disagreement on anything you want, from interpretations of doctrine to how it should be applied in real life. There are jews who care deeply about religion, and there are Jews who barely give a **** about it. Jews are no more culturally monolithic than Christians.
If you wanted Dalish as culturally united as Jews, you got that.
If you wanted Dalish as culturally united as the antisemtic creation of the monolithic jewish archetype, but different, you were kidding yourself.
Now when a problem arises between Arlathvhens that their lore doesn't seem capable of dealing with, the Keeper and the clan are going to have to make the best of it and then refer back to the gathering of the elders (Hahren'al) for their opinion on the matter. This is what happened with the issue of Zathrian when it came to light. They discovered that after all these years he hadn't discovered the secret of their immortality but in fact had undertaken a forbidden ritual using manipulation of life and blood. The elders then condemned this action as a crime against nature. They understood why he had done it, but they did not condone it. So if a Keeper turned up down the line doing the same thing, this would not be in keeping with Dalish customs as established by the Hahren'al.
The Hahren'al doesn't establish what Dalish customs are. What Dalish actually do the other 9.9 years of the decade establishes Dalish customs are. If Keepers (plural) down the line keep doing the same thing, it is a Dalish custom no matter what the Hahren'al publicly espouses. Your cultural practices are what your culture practices on a regular basis, not what it claims it will practice once a decade or so.
Legally, blood magic is forbidden in Tevinter. Culturally, it's a tolerated and ongoing practice, whether it's publicly acknowledged or not.
I repeat the actions of Zathrian were not that of a typical Dalish and the clan leaders condemned it at the next Arlathvhen.
Political gathering condemns unpopular thing after it fails disastrously. Shocking!
What Marethari did in DA2 was only typical in that Merrill was banished for refusing to give up magic that was specifically outlawed under Dalish tradition. Much of the time she very much acted against Dalish tradition in persistently putting her clan at risk for the sake of a person who had left the clan through her own choices.
A Keeper risking a clan as an extension of their own biases and concerns is an established Dalish tradition in the clans we see. They're an authoritarian system which amplifies the authority's personal biases, whatever they are.
Nor was Merrill banished for defying Dalish law. Dalish has no law, or court to appeal. She was banished for defying Marethari's directions. She would not have been banished in a different clan whose Keeper disagreed with Marethari.
As for the narrative in DAI, let alone in Masked Empire, that is the whole problem. The writers are increasingly making the Dalish appear less sympathetic to the player so setting them up to be destroyed without anyone being particularly upset over the matter. That's what PW did in Masked Empire in making such a extreme caricature of Dalish belief. They even made it far easier for Lavellan to get their clan killed than have a successful outcome. I'm pretty sure the Dalish PC is the only one who can potentially lose their entire family in a war table mission.
Alternatively, since you seem to have a problem with every single instance in the media of the Dalish presentation, the problem is actually that you mis-understood the Dalish from the start, and each new iteration continues to fail to validate it.
It's fine to be upset that the Dalish aren't what you wanted them to be- the idea of the proud, noble savages heroicly resisting injustice as they valiantly struggle for what should be rightfully theirs, a group whom you wanted to restore to their lost claimed glories. There might have been a good story in that. But it doesn't make it bad writing if that was never the story Bioware wanted to tell with the Dalish.