It's not just the spiritual side either. Traditional knowledge is also about stuff like exactly how much to use, how to prepare it, what to use it with (and not to use it with), what time of day, beneficial activities before and after, and so on. The human body is a complex system where all of these things cause complex interactions that can influence efficacy. Traditional medicinal use is full of these kinds of 'full package' treatments, a lot of which is lost when you just bottle some ingredient.

I think it depends. In Brazil it's common (and lawful) to consume Ayahuasca in certain religious contexts. The biggest religions are Santo Daime and União do Vegetal - both are heavily Christian.

I know personally several people, many of whom hardcore atheists, who have had life-changing experiences taking part of those rituals and many have gone back or made it a regular occurrence. None of them ceased to be hardcore atheists.

Yes. And in this case consider the ritual a part of the recipe, context, necessary environment for a certain complex healing experience.

Tested and iterated on through the years, and if your worldview allows for the concept of plant consciousness, consider that the steps may have been provided by its suggestion.

If one’s worldview is 100% materialistic, an encounter with some of these plants may change that.