Cult - a religion devotedly followed by a small group. A movie that has become a "cult film" is one that a small group of fans watch over and over again.
Pan - is used as a prefix here, I would have written pan-Indian - from a Greek word meaning everything. (Think of Pan-Am Airlines.) The writer means that Amitabh is popular all over India.
"to being" - "being a Tamil icon" is a gerund phrase that is the object of "restricted to." The patient can't eat candy; he is restricted to a no sugar diet.
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