Question
On the nature of Krishna
Answer
Krishna was already a "realized". He's the ever-realized for he's Vishnu himself, or Krishna (of Goloka) himself.
This is what he says in the Śrīmad-Bhāgavatam Purāṇa when he "departs" from his mortal coils when hit by the hunter Jarā.
श्रीभगवानुवाच
मा भैर्जरे त्वमुत्तिष्ठ काम एष कृतो हि मे । याहि त्वं मदनुज्ञात: स्वर्गं सुकृतिनां पदम् ॥ ३९ ॥
Sri Krishna said: My dear Jarā, do not fear. Please get up. What has been done is actually My own desire. With My permission, go now to the abode of the pious, the spiritual world.
So the answer to your question 1 & 2(a) is, he was a realized person.
As regards question 2(b) Vishnu being present in his abode, when he descended as Krishna, yes. He was very much present in the Svetadvipa - Ksheer Sagara, as we can see in this excerpt from Śrīmad-Bhāgavatam Purāṇa where Krishna (along with Arjuna) meets Vishnu.
ववन्द आत्मानमनन्तमच्युतो जिष्णुश्च तद्दर्शनजातसाध्वस: । तावाह भूमा परमेष्ठिनां प्रभु- र्बद्धाञ्जली सस्मितमूर्जया गिरा ॥ ५७ ॥
Lord Kṛṣṇa offered homage to Himself in this boundless form, and Arjuna, astonished at the sight of Lord Mahā-Viṣṇu, bowed down as well. Then, as the two of them stood before Him with joined palms, the almighty Mahā-Viṣṇu, supreme master of all rulers of the universe, smiled and spoke to them in a voice full of solemn authority.
Further,
- For question 3, it's well established that Krishna is Vishnu himself, and/ or the Original person of the Spiritual Goloka.
For the "I" in Gita, refer these -
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