Is it permissible to attend a Christian funeral without participating in their rituals, and is it obligatory to re-wash the deceased if his wife washed him by pouring water on his body without knowing how to perform the ritual washing?

It is not permissible for a Muslim to participate in the funeral procession of a disbeliever (kafir). It is also not permissible to re-wash a deceased person if his wife had already ensured that water covered his entire body during the washing, because the minimum requirement for a ritual bath is the complete coverage of the body with water. It is permissible for a woman to wash her husband.