“Saddhā” in the Buddha, Dhamma, and the Saṅgha comes though the comprehension of the true nature of this world (anicca, dukkha, and anatta.)
I have re-written an old post: “Sotapatti Aṅga – The Four Qualities of a Sotāpanna”
Answers to your question, including Q4, are in the above post.
“Kathaṁ vicikicchā na bhavati?” in Q4 means “How does one remove any doubts (about Buddha, Dhamma, and the Saṅgha)“?
– True saddhā comes via removing any doubts about the teachings of the Buddha, i.e., the “wider worldview of the Buddha.” With that understanding, one can “see” how future suffering can be eliminated.
– A huge relief to the mind comes from that, a type of nirāmisa sukha.
Of course, feel free to ask questions to clarify any issues.