What parts of the visual system could be responsible for a fixed, monocular scotoma?
Yes. You're right.

A scotoma only affecting one eye (i.e., one that is not perceived when one eye is closed) is diagnostic of a lesion upstream from the optic chiasm. 

See Brust's Practice of Neural Science, Fig. 3-2. The figure demonstrates a full cut, with complete blindness in one eye, but the principle is the same for a scotoma.