Non fui, fui, non sum, non curo
on adjacent squares, yes they can see each other in both cases afaik.
The LOS is blocked if such a space(s) is(are) only in between of two chars..
I don't believe this is correct. In the case of smoke (which is a different case from obstacles that block LoS), LoS is blocked in the area of effect. This would mean that characters standing in the smoke cannot be seen even if adjacent.