Can severe neck/shoulder/posture problems and pain cause facial/head neuralgia, vision disturbances and dizziness? docs said migraine but it's been 3 weeks now?

Possibly: Cervical spine issues can be the trigger for migraines due to pain, pinched nerves as well as muscle spasm. You can also develop trigger points where scalp muscles originate or nerves can get pinched as they travel through layers of muscle. Chiropractic and massage can be helpful in controlling migraines triggered this way after determining no serious structural neck findings.

Yes it can: By causing pressure points on certain areas of the nerves that supply neck and face that leads to shooting pain in those areas.

See a neurologist: The symptoms you described is a serious neurological condition and an appointment with a neurologist for neurological evaluation and imaging is warranted.