How famous is Lotus Dharani amongst East Asians compared to Casket Seal Dharani and Mahacundi Dharani? Do you have any commentary on Lotus Dharani?
Since other Dharanis are more famous amongst the Chinese, Taiwanese and Japanese, I guess Lotus Dharani must be one of the more difficult ones which require greater prerequisites for practicing. Ou-I, the Buddhist monk, who wrote Buddhist I Ching, said that for lay disciples all the mantras and dharanis are risky (I wonder why he said so, though I guess it may have to do with the strength of precepts maintaining of the individuals), and only the Nianfo is the safest. Which is why I practice Nianfo. But recently, I have been reading more about the Mantras and Dharanis from other Mahayana scriptures.
Coming from a Chinese Buddhist background, my observation of the most famous and widely-practised/upheld dharani/mantra would give me this list:
Great Compassion Dharani
Om Mani Padme Hum
Medicine Buddha Mantra
Casket Seal Dharani
Hundred-Syllable Mantra of Vajrasattva
Usnisa Vijaya Dharani
Great White Umbrella Dharani (my rough translation)
Great Wish-Granting Dharani (my rough translation)
There might be a few more that could make it to the list but I can't think of right now. However, Lotus Dharani would not be one of them. I know it is very popular amongst Chinese Buddhists to chant the whole Lotus Sutra, but not specifically the Lotus Dharani.
In terms of a desirable level of purity to be maintain by a practitioner to use and uphold these dharani, I have read that even though the sutra may not explicit state them they are implied to greatly help the dharani practice. For daily practice by a lay person, purity would mainly consist of upholding the Five Precepts and a vegetarian diet (including no egg and pungent vegetables like garlic, onion, chives, etc.).
The Cundi Dharani is the exception, the sutra of which explicitly states this practice is also suited for people who cannot immediately refrain from e.g. eating meat, drinking alcohol, having a marital relationship.
I have recently read that the Ten Hearts as taught in the Great Compassion Dharani Sutra are also applicable to all dharani/mantra.
I agree that Nianfo is very safe. The practitioner's purity would be helpful too, but probably not critically essential as I understand it. I would extrapolate the teaching about the Ten Hearts to be applicable to Nianfo also. Nianfo has the additional benefit that one could practise Chan/Zen using it, whereas I have seldom heard people do that with dharani/mantra practice.