Is someone who dies from an electric shock directly to the heart considered a martyr, even if no external burns are visible? Is the feeling that he is still alive considered a sin, and how can one get rid of it? Is the supplication of relatives for the deceased considered an ongoing good deed for him, like the supplication of children?

Whoever dies by electrocution is not considered a martyr, unless the electrocution leads to the body being burned, in which case they would be a martyr of burning. And sudden death is a comfort for the believer.

As for your feeling that your uncle is still alive, there is nothing wrong with it as long as you do not speak or act inappropriately, and it is likely due to your strong attachment to him.

The supplication of those other than one's children for the deceased is not considered among the deeds whose reward continues after death, because a child is among the earnings of their father. However, the deceased benefits from the supplication of others for them, whether relatives or strangers.

The hadith, "When a person dies, all his deeds come to an end except for three: a continuous charity, or knowledge from which benefit is derived, or a righteous child who prays for him," means that the supplication of a child is from the deeds of the deceased whose reward does not cease. As for the supplication of others, it is not from his deeds, but he benefits from it.