All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
A person who memorizes the whole Quran and acts upon it gets many benefits, some of which are mentioned in Fatwa 85360. Among other benefits is that he will get what is mentioned in the saying of the Prophet "Whoever reads the Quran, memorizes it, and acts upon it, on the Day of Judgment he will be clad (by angels) with a crown of light, its light is like the sunlight and his parents will be clad with two garments better than the whole world and whatever it contains." So they would amazingly ask: "What action did we do to deserve this?" They will be told: "Because your son memorized the Quran". [Al-Haakim] There are other benefits that no one knows but Allaah.
As regards whether or not it is sure that a person who memorized the Quran will enter Paradise, the answer is that whoever memorizes the Quran for Allaah's Sake and acts upon it, then this is one the greatest reasons of being admitted to Paradise, but if one memorised the Quran [without acting upon it], it is not sure that he will be admitted to Paradise. Indeed the Prophet informed us that the first people to be thrown to Hellfire on the Day of Judgement are three people. One of whom is a person who memorized the Quran. Allaah will remind him, "Did I not teach you what I revealed to My Prophet ?" The man would reply: "Truly." Allaah will then ask him: "What did you do with what you have learnt?" The man would reply: "I used to act upon it during the day and night." Allaah would say to him: "You are a liar.", and the angels would say to him the same thing. Allaah would say to him: "You memorized the Quran so that people would say you are a Haafith [a person who memorized the Quran well and they already said so." [At-Tirmithi and Al-Haakim] So, this person did not learn the Quran for Allaah's Sake so his punishment is that he will be the first person to enter Hellfire. Therefore it is not definite that whoever memorizes the Quran will enter
With regard to whether or not a person who memorized the Quran will intercede for his relatives, the answer is that if he acted upon it then he is among the pious people. Indeed many Prophetic narrations state that believers will intercede for each other on the Day of Judgement. A person who memorized the Quran [and acted upon it] and a scholar will Inshaa' Allaah intercede for their beloved Muslim brothers. However, we do not know any authentic narration which states a definite number of people for whom they will intercede except one narration reported by At-Tirmithi with the following wording: "A person who memorized the Quran, adhered to that which is permissible, and refrained from that which is forbidden, Allaah will admit him to Paradise and allow him to intercede for ten people of his family, all of whom deserved the fire." The chain of narrators of this narration is weak. Al-Albaani said about it: ''This narration is very weak.'
With regard to whether or not it is a great major sin to forget the Quran after having memorized it, the answer is that forgetting the Quran is negligence and a sin. Shaykh Al-Islam Ibn Taymiyyah, may Allaah have mercy on him, said: 'Forgetting the Quran is a sin.' Many righteous predecessors considered forgetting the Quran as a major sin. Ibn Hajar said: 'Our righteous predecessors differed about forgetting the Quran, some of them considered it among the major sins.' Ibn Seereen, may Allaah have mercy on him, said: 'They [righteous predecessors] used to dislike a person who forgets the Quran and severely rebuke him.' However, Ibn Rushd from the Maaliki school, reported that there is a consensus among the scholars that a person who forgets the Quran because of being preoccupied in acquiring an obligatory or permissible knowledge, is not sinful. We do not know any authentic narration which states any punishment for a person who forgets the Quran.
As regards the narration that you mentioned at the last part of your question, we have not come across it in any books or resources that we have despite their great number; it is only Allaah Who knows about it.
Allaah Knows best.