I don’t think anyone can speak on behalf of Vaheguru. He is completely independent Being but full of mercy. So one thing we can be sure of is that He is forgiving and there isn’t anything that He cannot forgive.
That being said, the method of forgiveness is different than other religions. Gurmat does not believe that by simply asking for forgiveness, one will get it without ever reforming himself. Any sin we commit, minor or major, we incur bad karma and it leaves that imprint on our mind. More bad karma makes our mind dirtier. Naam, on the other hand, cleans it.
So if a person wishes to be forgiven from a major sin (adultery etc.), two things are required, Rehat and repentance. By repenting one stops himself from committing the same mistake again. But this alone is not enough because the filth has already been incurred and cannot be cleaned by regretting alone. One must transform himself and develop himself to become a better human. So one must abide by rehat and strive to do significant amount of Simran so that he can burn his karma faster especially the bad ones. This will lead to cleansing of the mind and becoming a better servant of the Guru. This would eventually bring about mercy of Vaheguru and the punishment of the sin would be reduced significantly. The punishment might come in form of some physical pain or a brief visit to hell during sleep. Or it could be forgiven altogether. This is all in the hands of Vaheguru. But the person will not have to go to hell after death. Bhai Rama Singh went through this same exact situation where his punishment for adultery was delivered during his sleep.
Every sin is different and therefore, the situation will be different. A person’s lifestyle is a major contributing factor. And there is no rule that binds the actions of Vaheguru and His mercy. Bhai Rama Singh was not an Amritdhari when he was taken to hell but an Amritdhari person will most likely not be taken to hell. So, all sins can be forgiven but a person must do his part to earn the forgiveness. Even then Vaheguru cannot be forced nor can any mediator make pleas. I do not say I am 100% correct. Guru Rakha