What is the ruling on marriage over the phone accompanied by masturbation and revealing the body to the fiancé via an internet camera, and how does one seek forgiveness for that?

It is not clear what is meant by "marriage over the phone."

If the marriage contract (Nikah) is concluded, fulfilling its Shariah conditions (guardian and two witnesses), then what is permissible between spouses becomes permissible for them. However, it is not permissible to expose the body and engage in sexual activity over the Internet, because this may lead to being seen and filmed by others. Also, this practice falls within the scope of prohibited masturbation if each of them performs it for themselves.

Before the marriage contract, each of the fiancés is considered a stranger to the other, and any of these practices are forbidden between them.

If "marriage over the phone" refers to the marriage contract itself, then it is not valid, as ruled by the Islamic Fiqh Council.

There is no specific worldly punishment (Hadd) for these actions, but rulers may impose a discretionary punishment (Ta'zeer) on the perpetrator. As for the punishment in the afterlife, it is part of the unseen.

Whoever has committed any of the aforementioned actions must repent to Allah by ceasing the act, regretting it, resolving not to return to it, and frequently seeking forgiveness.

The fiancés must immediately sever their relationship before the marriage contract until the legal marriage is concluded.