Vishwamitra - as per Devi Bhagwad Purana

There was king called Vishwamitra, who one day, while roaming around came across his kingdo, accidentally ended up at the hermitage of rishi Vasistha. The king bowed before the rishi, who invited the king and his followers for a meal. The sage had a cow, Nandini, from whose milk the sage prepared all the food for the king. The king wanted the cow, but the rishi refused to give it away. THe king offered a thousand cows in exchange, even that was refused. He even offered a million cows which was also refused. The king said he would carry the cow over by force.
The king tied a rope around the cow's neck and tried pulling it away. The cows bellowed loudly and two demons were born who attacked and destroyed the king's followers.
Humiliated and dejected, the king went off to the forests where he spent his days in meditation and later became a rishi.

Legend of Sakhachuda - From the Devi Purana

Yesterday, November 23, was the day for Tulsi Vivah here. I had heard of Jalandhar and Tulsi, but the Sankachuda story was new. Searching through the Devi Purana, I found this.

There was a pious king called Dharmadhwaja.He and his wife Madhavi had a beautiful daughter Tulsi. When Tulsi was a young woman, she left and went to the Badrika where she started meditating on Narayana to get him as her husband.
In her earlier birth she had been a Gopi in Gokula who desired to have Krishna for herself. She had been cursed to be born as a mortal woman. Radha had cursed her as love for Krishna was without physical attachment. Sudama, friend of Krishna had witnessed this cursing and had told Radha that her curse was justified. Furious, Radha had cursed Sudama to be born as Sankhachuda, a demon.
When Brahma appeared before Tulsi and asked what she wanted, Tulsi said she wanted Narayana as her husband. Brahma said Sudama had wanted her and as the demon Sankhachuda he still desired her. Meeting the demon, Tulsi discovered that she too liked him and the two got married.

Like other demons, Sankhachuda also attacked the Gods and tried to take over the heavens. He had an invincible armour called Sarvamangala, that was protecting him, As long he wore that, he was invincible. It had been given to him by Narayana, who had been pleased by the demon's meditation.

The only way Sankhachuda could be killed was if his wife lost her chastity. That would destroy the armour.

Narayana (Vishnu) approached Tulsi, when Sankhachuda was himself in the battlefield with Bhadrakali. He first prayed to Shiva and Bhadrakali and then taking their blessing started the war. In the meanwhile, seeing Narayana in front of her, Tulsi lost herself and Sankhachuda's armour was weakened.
Narayana went back to the battle field and there Shiva struck Sankhachuda.
Taking the appearance of the demon, Narayana then approached Tulsi. Tulsi wanted to know all from whom she thought, was her husband. Later realising something was not right, she wanted to know who he was. Then when Narayana revelaed himself, Tulsi cursed him to become a stone.
Vishnu (Narayana) then blessed her saying she will be born as a plant, Tulsi, and no worship of Vishnu would be complete without the leaves of Vishnpriya or Tulsi.
Narayana was then transformed a part of himself as a Shalgram stone. 

Ramayan in other sacred texts

I am coming across references to the Ramayana in many other texts.
Srimad Bhagwad Purana has a reference to Rama and killing of Ravana. This book takes it further and talks about descendants of Rama.

Then there is Devi Bhagwad Purana. This Purana talks about Devi and her greatness. This Purana incidentally also has Ramayana here.This also talks about how Rama performed Navaratri and worshiped the Goddess.

This story of Rama praying to the Goddess does not find a mention in the Valmiki Ramayana, but is there Krittibasi Ramayan, the Bengali version of the epic.

This also gets to think about Ramayana. As this is mentioned in the various texts, was Ramayana something that had happened.

Also, who was Veda Vyasa, the author of Mahabharata and the Puranas? Was this a person  or a group of people, who knew the Vedas and had mastered them (A-Bhayas), thus getting a title Veda Vyasa?

No clue.

Its good to read and trying to understand and at the most, enjoy them.