Used exploratory mixed methods. Start with qualitative research. Then use the results to generate a survey in the second phase of your study

Depends on whether you'd like to conduct such survey only in one religion or in various religions.

You may like to decide which religions will be included in your research, and ask both the leaders and the subjects on their methods and the psychological challenges that they tried to resolve.

The more detailed information you can provide for each of these leaders, the better. You may like to reach out to HWPL South Korean organization that conducts interreligious discussions in over 70 countries of the world. They may like to connect you with various religious leaders who could provide you all information in your survey.

What do you want to measure? That which is immeasurable: the qualities of intuition, imagination, insight, that which is individual, unique? Or perhaps it is worth posing the question differently: what will classical science accept of the spiritual if this is lacking in the current paradigm? Maybe you should think about a paradigm shift first, because the glass and eye methods are not adequate to the spiritual level?

No, spiritual leader is not a professional on the aspect, what is not measurable in cannot have total value in any ramifications

it is always back to what how you want to conduct and what are the variables you want to choose. Maybe you can look at Prof.John W.Fisher on Domains of spiritual well-being and development and validation of the Spiritual Well-Being Questionnaire, where it proposed a spiritual well-being model, comprising the domains of personal, communal, environmental and transcendental well-being, and a single global spiritual well-being dimension, which you can see how much all the domains can relate to its outcome if the spiritual leaders in their own personal and the community, effect their spiritual well-being