Children

It is narrated from the Prophet Muhammad (S): “Never beat the children if they cry. Fulfill their needs. Because, for the first four months of the life of a child, its cries are a witness to the Existence and Unity of Allah, Almighty." (Bihar al-Anwar)

In general, in Islam, one should avoid breaking ties with one's family unless there is a serious and overriding reason (such as fear for one's life). However, whether one wishes to maintain a close relationship or a more distant relationship is a personal choice.

I don't know the details of this situation. I would surmise that in most cases, if parents break ties with their children on account of their child's poor academic performance, it is due to (a) other issues, not just academics,  (b) an attempt to motivate the child to do better by "punishing" them, or (c) a projection of their own psychological issues onto the child. 

Anyway, all the child can do is try to be polite and respectful, try to avoid arguments, and keep the door open in case they wish to resume communication. 

If a young person is not doing well in academics, and has tried various ways to improve but has not succeeded, it is worth considering other options such as vocational school or going into business. Everyone has their own skills and talents. There are a lot of jobs that are not related to academics (such as being a chef or plumber) which pay well and are necessary jobs for society, even if the parents' dream is for their child to do something else. 

There are advantages and disadvantages to marrying closely (like a first cousin). Sometimes the advantages, such as having greater trust, might outweigh the disadvantages. To my understanding, occasional close marriages are not usually a problem with genetics, rather, problems tend to arise when people do this over a long span of generations. Also they have been common in many cultures not just among some Muslim cultures. Genetic problems can also arise in any marriage. However there is nothing in Islam recommending this particular type of marriage. 

I one saw someone teaching Surah al-Rahman by associating meaningful signs with each of the verses, so, for instance, in this verse, they pointed to stars and made trees with their hands and fingers. The idea behind this is that the human being will remember gestures and words more than gestures. You could try it!

The hardships and tests we face - God willing - refine us not only as human beings but increase our spiritual understanding and capacity to behold the divine. This is one reason why human being agreed to accept the responsibility of free will - with it comes a risk (hellfire) but also great potential.

Ideally the main goal of the material existence (as I understand it) is for human beings to deepen their inherent spiritual understanding as much as possible. A child and an adult may both go to jannah; however, depending on their experiences (as not everyone grows throughout a long lifespan), they may end up with a different inherent capacity to perceive the divine there. (One might equate that with "higher in jennah")