Hinduism--i have some questions on this interesting religion

Very pertinent questions. I will try to answer them to the best of my knowledge. It is right that when you attain nirvana, you become one with God. Nirvana is more of a Buddhist term. It is the ultimate state of realization and its equivalent term in Hinduism is Nirvikalpa Samadhi. Even this Nirvikalpa Samadhi has two states - Kevala and Sahaja. In Kevala Nirvikalpa Samadhi, your ego, "I"ness, or your mind dissolves into Brahman and as long as you are in that state, you have no individuality of your own. Sri Ramakrishna Paramahamsa says that if you remain continually in that state of merger for 21 days at a stretch, you will relinquish your body.You have attained your goal and you will not have rebirth. This is the case of highly qualified and earnest seekers of spiritualty, whose "capacity of acquiring and retaining divine power" is somewhat of a lower order. Sri Ramakrishna calls them "Jivakotis". If such jivakotis, after experiencing the bliss of the union for a while, regain their consciousness to the world, their ego too returns (though it will be very purified and subtle). It means, their "I"ness returns and along with it, "you" and "others". It means, from an experience of non-duality, they come back to the plane of duality once again. Their state is thus not permanent (Kevala). They are really great souls, no doubt about it; Having seen and experienced Brahman, they are far ahead of mundane spiritual leaders and teachers. Though they have had the greatest illumination, their experience is not permanent. They are to be respected and highly revered. No question about it. Then you have the category of Ishwarakotis. Avatars (incarnation of God) and very close and direct disciples of Avatars -- those who come to earth along with the avatars to assist him in his mission are the Ishwarakotis, according to Sri Ramakrishna. They attain Nirivikalpa samadhi in their life (i.e. the experience the union with Brahman, i.e. non-duality); their experience gets "solidified"; When they regain consciousness to the world again, they do not regain their ego; they do not see the world and creations with a dualistic view. They see everything verily the manifestation of Brahman; If they see their body, what they see is Brahman; if they eat, it is Brahman that eats; if they act, it is Brahman that acts through their body. They have no sense of doership. Brahman and their atman are one and the same. Hence their atman is nothing but brahman. There state is Sahaja (ever existing) Nirvikalpa Samadhi. If they seem to have an ego, it is like a burnt rope. A burnt coir rope retains its shape in cinder form. It looks like rope, but if you blow air on it, everything flies away. Thus, in an Ishwarakoti, others may see some existence of Ego or mind, but he knows he is and he has nothing but atman. He is all at once a Gnyani (knower) and a Bakta (devotee of God). Depending upon his basic bend of mind originally, he may predominently display a Gnayani mentality ("I am Atman") or Bhakta mentality (a great lover and worshiper of God -- "I am nothing; only You exist" or both. He is both the worshiper and the worshiped! Depending on individual mental maturity and spiritual development, each believer sees him either as a Gnayni, a God, a Satguru (spiritual master), a God incarnation, or a great Bhakta. He is all of these; yet, he is beyond all of these! This is not a state that can be explained by words. No amount of theorizing can help explain this. It is a state to be experienced. Regarding your second question, I will write again separately.