Could one have multiple sclerosis with a normal head MRI?

Yes. Or at least have clinically isolated syndrome with optic nerve or spinal cord involvement. (Devic Syndrome-neuromyelitis optica is a demyelinating condition that can convert to MS.
Yes and no. MS is an inflamatory disease of the central nervous system. It can affect the brain and spinal cord or only one or the other. you can have lesions in the spinal cord but not the brain, although this is rare. Usually you do have some brain lesions.
Unlikely. Although unusual, sometimes MS shows up on spinal cord MRI initially, without abnormal brain MRI, but your normal films most likely represent another diagnosis.

Related Questions

Had MRI for headaches. With & without contrast. Normal. Would MRI also show anything else. Such as multiple sclerosis?

Normal. If "normal" then you do not have multiple sclerosis. You may have migraines. I see numerous migraine patients with normal MRI studies. Best course of action is to talk to the doctor that ordered the MRI and talk over a plan. Read more...
If disease present. MRI should reassure you, as lesions characteristic of MS are not noted and there is no sign of bleeding or MS. Read more...