Is it forbidden to shave the beard that grows only on the sideburns? Is it forbidden to trim the beard in general by cutting it slightly to be somewhat even? What is the limit that permits cutting the hair of the sideburns if it exceeds a fistful?

It is forbidden to shave any part of the beard, including the hair of the sideburns (al-'idhaar), which is considered part of the face and falls within the beard's boundaries. It is not permissible to trim the beard unless there is a clear and harmful disfigurement, in which case it is permissible to trim from the dense side to remove the apparent disfigurement and to make the hair on both sides more uniform.