Quix, a doctor who used to post here, has covered this topic a number of times. Here is one link.
Please note though, her response here takes into account the poster's significant eurological signs and symptoms. Your neurologist will as well. Nothing you have posted as far as symptoms (isolated feeling of falling; migraine onset) could be directly attributed to ms.
Has your husband had a neuro exam? Your pcp could do a basic one.
What did the impressions or conclusions section of his report say?
Research shows smoking can cause white matter changes in subcortical and periventricular areas. I'm not sure about the other areas you mentioned. Without a positive neuro exam, the MRI results might be blamed on your husbands's extensive smoking history. This may in fact be the cause. The increase due to family history us actually quite small, although I can't remember the value.
It will definitely take a neurologist to sort out the MRI and your husband's history because the chances are much higher that the damage is ischemic due to smoking.
For those with ms, smoking is linked to increased symptoms and faster progression to secondary progressive.
I've been exposed to chemicals like those found in cigarette smoke via occupational exposure (on a large scale) and do expect (confirmed by my neurologist) that this was my environmental trigger for developing ms, even though there is not one known "cause" of ms. As well as acute toxicity, chemical exposure can cause immune dysfunction.
Best wishes as you and your medical team sort through this. You husband will likely have to have MRAs done, and a cardiac work up, as well as the more typical rule outs.
Keep us posted!