Q1- What is the role of Samadhi (Super Consciousness) in Vedanta ?
The word Samadhi is from yoga tradition. Major Upanishads commented by Shri Shankara do not mention Samadhi. However, word Samadhi is found in Yogic Upanishads of later age. The closest word for samadhi in Upanishads is Samahita (Collected mind) which was later called Samadhana.
Word Samahita means composed, or collected mind. A spiritual aspirant should develop four fold qualities required for knowledge of SELF (Brahman) called Brahma Vidya. They are Viveka ( discrimination), Vairagya (detachment), Shama ( calm mind), dama ( control over sense organs), Titiksha ( ability to bear pain), Uparati ( mind focused on SELF knowledge, Shraddha (faith) faith in Guru and teachings of Vedanta, Samadhana (Samahita) -Collected state of mind and mumukshatva ( desire for liberation).
Samadhi in yogic literature is not same as Samahita in Vedanta. There is no mention of suspension of faculties in Vedantic literature. Samadhi is specific to yogic practice.
Q2- Is Nirvikalpa Samadhi necessary for Self Realization according to Vedanta ?
No. Samadhi is suspension of faculties of mind. Nirvikalpa samadhi is suspension of thoughts. Thoughts being objects, Nirvikalpa samadhi is a state of objectless consciousness. Samadhi is time bound. The ignorance is not destroyed in samadhi and hence samadhi is not required for Self Realization. Self Realization in Vedanta is dropping the ignorance (avidya). Mind is a product of ignorance (avidya). Avidya can only be removed through vidya.