Actually, that is the only way it does happen... It starts with an individual within a group of individuals and is introduced into parent, peer, or child groups.
For a ceremony/custom/ritual to continue and thrive in each group that it is introduced into, these groups of individuals must find some meaning or benefit from engaging in the activity.
Not quite. What happens is that a bunch of individuals within a culture independently come to the same conclusion based upon the pre-existing culture they have in common (same thing applies to ideas as with customs). Then they realize that they have something in common and create an institution. The institution then spreads the custom to everyone in the culture, regardless of whether they would independently develop an interest in it on their own. This is why cities and urban centers drive culture forward so much, because in a city it is easier for a number of individuals as described above to meet.
You are saying that trees (which exist above-ground and below-ground in DF) are not part of their life... but tea leaves (from one type of tree that only grows in tropical regions), are part of their life.
The tea leaves come from the surface because they are harvested there (in real-life tea trees are actually high-altitude plants, they will grow happily in Scotland) and brought below by a section of the population. However we are talking about literal tea there, nothing keeps there from being other plants, even underground ones of which 'tea' can be made; perhaps with more industry being involved than with tea leaves.
About trees, the point is kind of that trees lack general relevance to the population. Some dwarves will find trees relevant, because they go there to harvest things from trees, whether they be fruit, tea leaves or wood. But they don't have general familiarity to everyone, which means their cultural significance would remain for traditional fantasy dwarves limited; this does not have anything to do with their abstract ideas about nature.
However if you stretch things a bit we can end up with a different situation to traditional fantasy. If we have a dwarf fortress below a forest and nearly everyone goes out to get fruits once a year, trees could become a key cultural thing.
Fermentation continues until the sugars have all been converted to alcohol, and dwarfs are not involved in this activity once the container is sealed. I agree that it takes less effort to make tea than to make alcohol, since fermentation requires the effort of putting the sugars into the container and if the dwarfs live in a biome with tea trees, it is easier to bring a bucket to the pond full of rotting tea leaves and fill the bucket.
I was not talking about the effort involved
. I was talking about the sugars and carbohydrates used up by the production of alcohol. Traditional fantasy dwarves live in mountainous areas underground, which amounts to two environments where energy is scarce. Above them there are few carbohydrates and below them (in the caverns) there are also few carbohydrates, most energy in those environments is the protein and fats in the animals of creatures. There simply isn't the ability to mass-produce beer or wine as there is in human societies, because dwarves will simply have to eat the scarce carbohydrates and sugars needed to make them in order to stay alive.
Tea on the other hand, that grows in mountainous slopes as it's idea environment. Between alcohol and tea, tea makes more sense because this frees up all the energy that would be used to feed yeast in order to feed dwarves.